In order to increase our own personal merits before God, we must first of all be in the state of grace. If we are not in the state of grace, we are by default in the state of mortal sin. Simply by going to sacramental confession will restore us to God’s friendship and make us capable of gaining more merit again in the eyes.
St. Thomas Aquinas has a solid perspective to this issue at hand:
Once in the state of grace, one can increase one merit before God in many ways. But the degree of merit will vary by degree and each individual will not merit the same amount before the Lord as each person is so unique and the amount of supernatural charity with which a person who performs a particular meritorious action will also vary.
That said here are a few things one could do in order to gain as much merit as possible in the eyes of God:
The list is fact seemingly endless, so I willed here and let the Catholic Encyclopedia do the rest.
Conditions of merit
For all true merit (vere mereri; Council of Trent, Sess. VI, can. xxxii), by which is to be understood only meritum de condigno (see Pallavicini, "Hist. Concil. Trident.", VIII, iv), theologians have set down seven conditions, of which four regard the meritorious work, two the agent who merits, and one God who rewards.
(a) In order to be meritorious a work must be morally good, morally free, done with the assistance of actual grace, and inspired by a supernatural motive. As every evil deed implies demerit and deserves punishment, so the very notion of merit supposes a morally good work. St. Paul teaches that "whatsoever good thing [bonum] any man shall do, the same shall he receive from the Lord, whether he be bond, or free" (Eph. vi, 8). Not only are more perfect works of supererogation, such as the vow of perpetual chastity, good and meritorious but also works of obligation, such as the faithful observance of the commandments. Christ Himself actually made the attainment of heaven depend on the mere observance of the ten commandments when he answered the youth who was anxious about his salvation: "If thou wilt enter into life keep the commandments" (Matthew 19:17). According to the authentic declaration of the Fourth Lateran Council (1215) the married state is also meritorious for heaven: "Not only those who live in virginity and continence, but also those who are married, please God by their faith and good works and merit eternal happiness" (cap. Firmiter, in Denzinger, n. 430). As to morally indifferent actions (e.g., exercise and play, recreation derived from reading and music), some moralists hold with the Scotists that such works may be indifferent not only in the abstract but also practically; this opinion, however is rejected by the majority of theologians. Those who hold this view must hold that such morally indifferent actions are neither meritorious nor demeritorious, but become meritorious in proportion as they are made morally good by means of the "good intention". Although the voluntary omission of a work of obligation, such as the hearing of Mass on Sundays, is sinful and thereby demeritorious, still, according to the opinion of Francisco Suárez (De gratia, X, ii, 5 sqq.), it is more than doubtful whether conversely the mere omission of a bad action is in itself meritorious. But the overcoming of a temptation would be meritorious, since this struggle is a positive act and not a mere omission. Since the external work as such derives its entire moral value from the interior disposition, it adds no increase of merit except in so far as it reacts on the will and has the effect of intensifying and sustaining its action (cf. De Lugo, "De pœnit.", disp. xxiv, sect. 6).
As to the second requisite, i.e., moral liberty, it is clear from ethics that actions, due to external force or internal compulsion, can deserve neither reward nor punishment. It is an axiom of criminal jurisprudence that no one shall be punished for a misdeed done without free will; similarly, a good work can only then be meritorious and deserving of reward when it proceeds from a free determination of the will. This is the teaching of Christ (Matthew 19:21): "If thou wilt be perfect, go sell what thou hast, and give it to the poor, and thou shalt have treasure in heaven."
The necessity of the third condition, i.e., of the influence of actual grace, is clear from the fact that every act meriting heaven must evidently be supernatural just as heaven itself is supernatural, and that consequently it cannot be performed without the help of prevenient and assisting grace, which is necessary even for the just. The strictly supernatural destiny of the Beatific Vision, for which the Christian must strive, necessitates ways and means which lie altogether beyond what is purely natural.
Finally, a supernatural motive is required because good works must be supernatural, not only as regards their object and circumstances, but also as regards the end for which they are performed (ex fine). But, in assigning the necessary qualities of this motive, theologians differ widely. While some require the motive of faith (motivum fidei) in order to have merit, others demand in addition the motive of charity (motivum caritatis), and thus, by rendering the conditions more difficult, considerably restrict the extent of meritorious works (as distinguished from merely good works). Others again set down as the only condition of merit that the good work of the just man, who already has habitual faith and charity, be in conformity with the Divine law, and require no other special motive. This last opinion, which is in accordance with the practice of the majority of the faithful, is tenable, provided faith and charity exert at least an habitual (not necessarily virtual or actual) influence upon the good work, which influence essentially consists in this, that man at the time of his conversion makes an act of faith and of love of God, thereby knowingly and willingly beginning his supernatural journey towards God in heaven; this intention habitually retains its influence as long as it has not been revoked by mortal sin. And, since there is a grave obligation to make acts of faith, hope, and charity from time to time, these two motives will thereby be occasionally renewed and revived. For the controversy regarding the motive of faith see Chr. Pesch, "Prælect. dogmat.", V, 3rd ed. (1908), 225 sqq.; on the motive of charity, see Pohle, "Dogmatik" II 4th ed. (1909), 565 sqq.
(b) The agent who merits must fulfil two conditions: He must be in the state of pilgrimage (status viœ) and in the state of grace (status gratiœ). By the state of pilgrimage is to be understood our earthly life; death as a natural (although not an essentially necessary) limit, closes the time of meriting. The time of sowing is confined to this life; the reaping is reserved for the next, when no man will be able to sow either wheat or cockle. Comparing the earthly life with day and the time after death with night, Christ says: "The night cometh, when no man can work [operari]" (John 9:4; cf. Ecclesiastes 11:3; Sirach 14:17). The opinion proposed by a few theologians (Hirscher, Schell), that for certain classes of men there may still be a possibility of conversion after death, is contrary to the revealed truth that the particular judgment (judicium particulare) determines instantly and definitively whether the future is to be one of eternal happiness or of eternal misery (cf. Kleutgen, "Theologie der Vorzeit", II, 2nd ed., Münster, 1872, pp. 427 sqq.). Baptized children, who die before attaining the age of reason, are admitted to heaven without merits on the sole title of inheritance (titulus hœreditatis); in the case of adults, however, there is the additional title of reward (titulus mercedis), and for that reason they will enjoy a greater measure of eternal happiness.
In addition to the state of pilgrimage, the state of grace (i.e., the possession of sanctifying grace) is required for meriting, because only the just can be "sons of God" and "heirs of heaven" (cf. Romans 8:17). In the parable of the vine Christ expressly declares the "abiding in him" a necessary condition for "bearing fruit": "He that abideth in me, and I in him, the same beareth much fruit" (John 15:5); and this constant union with Christ is effected only by sanctifying grace. In opposition to Vasquez, most theologians are of opinion that one who is holier will gain greater merit for a given work than one who is less holy, although the latter perform the same work under exactly the same circumstances and in the same way. The reason is that a higher degree of grace enhances the godlike dignity of the agent, and this dignity increases the value of the merit. This explains why God, in consideration of the greater holiness of some saints specially dear to Him, has deigned to grant favours which otherwise He would have refused (Job 42:8; Daniel 3:35).
(c) Merit requires on the part of God that He accept (in actu secundo) the good work as meritorious, even though the work in itself (in actu primo) and previous to its acceptance by God, be already truly meritorious. Theologians, however, are not agreed as to the necessity of this condition. The Scotists hold that the entire condignity of the good work rests exclusively on the gratuitous promise of God and His free acceptance, without which even the most heroic act is devoid of merit, and with which even mere naturally good works may become meritorious. Other theologians with Francisco Suárez (De gratia, XIII, 30) maintain that, before and without Divine acceptance, the strict equality that exists between merit and reward founds a claim of justice to have the good works rewarded in heaven. Both these views are extreme. The Scotists almost completely lose sight of the godlike dignity which belongs to the just as "adopted children of God", and which naturally impresses on their supernatural actions the character of meritoriousness; Francisco Suárez, on the other hand, unnecessarily exaggerates the notion of Divine justice and the condignity of merit, for the abyss that lies between human service and Divine remuneration is ever so wide that there could be no obligation of bridging it over by a gratuitous promise of reward and the subsequent acceptance on the part of God who has bound himself by His own fidelity. Hence we prefer with Lessius (De perfect. moribusque div., XIII, ii) and De Lugo (De incarnat. disp. 3, sect. 1 sq.) to follow a middle course. We therefore say that the condignity between merit and reward owes its origin to a twofold source: to the intrinsic value of the good work and to the free acceptance and gratuitous promise of God (cf. James 1:12). See Schiffini, "De gratia divina" (Freiburg, 1901), pp. 416 sqq.
The objects of merit
Merit in the strict sense (meritum de condigno) gives a right to a threefold reward: increase of sanctifying grace, heavenly glory, and the increase thereof; other graces can be acquired only in virtue of congruous merit (meritum de conqruo).
(a) In its Sixth Session (can. xxxii), the Council of Trent declared: "If any one saith . . . that the justified man by good works . . . does not truly merit [vere mereri] increase of grace eternal life, and the attainment of that eternal life — if so be, however, that he depart in grace — and also an increase in glory; let him be anathema." The expression "vere mereri" shows that the three objects mentioned above can be merited in the true and strict sense of the word, viz., de condigno. Increase of grace (augmentum gratiœ) is named in the first place to exclude the first grace of justification concerning which the council had already taught: "None of those things, which precede justification — whether faith or works — merit the grace itself of justification" (Sess. VI, cap. viii). This impossibility of meriting the first habitual grace is as much a dogma of our Faith as the absolute impossibility of meriting the first actual grace (see GRACE). The growth in sanctifying grace, on the other hand, is perfectly evident from both Scripture and Tradition (cf. Sirach 18:22; 2 Corinthians 9:10; Revelation 22:11 sq.). To the question whether the right to actual graces needed by the just be also an object of strict merit, theologians commonly answer that, together with the increase of habitual grace, merely sufficient graces may be merited de condigno, but not efficacious graces. The reason is that the right to efficacious graces would necessarily include the strict right to final perseverance, which lies completely outside the sphere of condign merit although it may be obtained by prayer (see GRACE). Not even heroic acts give a strict right to graces which are always efficacious or to final perseverance, for even the greatest saint is still obliged to watch, pray, and tremble lest he fall from the state of grace. This explains why the Council of Trent purposely omitted efficacious grace and the gift of perseverance, when it enumerated the objects of merit.
Life everlasting (vita œterna)is the second object of merit; the dogmatical proof for this assertion has been given above in treating of the existence of merit. It still remains to inquire whether the distinction made by the Council of Trent between vita œterna and vitœ œternœ consecutio is meant to signify a twofold reward: "life everlasting" and "the attainment of life everlasting", and hence a twofold object of merit. But theologians rightly deny that the council had this in view, because it is clear that the right to a reward coincides with the right to the payment of the same. Nevertheless, the distinction was not useless or superfluous because, notwithstanding the right to eternal glory, the actual possession of it must necessarily be put off until death, and even then depends upon the condition: "si tamen in gratin decesserit" (provided he depart in grace). With this last condition the council wished also to inculcate the salutary truth that sanctifying grace may be lost by mortal sin, and that the loss of the state of grace ipso facto entails the forfeiture of all merits however great. Even the greatest saint, should he die in the state of mortal sin, arrives in eternity as an enemy of God with empty hands, just as if during life he had never done anything, meritorious. All his former rights to grace and glory are cancelled. To make them revive a new justification is necessary. On this "revival of merits" (reviviscentia meritorum) see Schiffini, "De gratia divina" (Freiburg, 1901), pp. 661 sqq.; this question is treated in detail by Pohle, "Dogmatik", III (4th ed., Paderborn, 1910), pp. 440 sqq.
As the third object of merit the council mentions the "increase of glory" (gloriœ augmentum) which evidently must correspond to the increase of grace, as this corresponds to the accumulation of good works. At the Last Day, when Christ will come with his angels to judge the world, "He will render to every man according to his works [secundum opera eius]" (Matthew 16:27; cf Romans 2:6). And St. Paul repeats the same (1 Corinthians 3:8): "Every man shall receive his own reward, according to his own labour [secundum suum laborem]". This explains the inequality that exists between the glory of the different saints.
(b) By his good works the just man may merit for himself many graces and favours, not, however, by right and justice (de condigno), but only congruously (de congruo). Most theologians incline to the opinion that the grace of final perseverance is among the objects of congruous merit, which grace, as has been shown above, is not and cannot be merited condignly. It is better, however, and safer if, with a view to obtaining this great grace on which our eternal happiness depends, we have recourse to fervent and unremitting prayer, for Christ held out to us that above all our spiritual needs he would infallibly hear our prayer for this great gift (cf. Matthew 21:22; Mark 11:24; Luke 11:9; John 14:13, etc.). For further explanation see Bellarmine, "De justif.", V, xxii; Tepe, "Instit. theol.", III (Paris, 1896), 258 sqq.
It is impossible to answer with equal certainty the question whether the just man is able to merit in advance the grace of conversion, if perchance he should happen to fall into mortal sin. St. Thomas denies this absolutely: "Nullus potest sibi mereri reparationem post lapsum futurum neque merito condigni neque merito congrui" (Summa Theol., I-II, Q. cxiv, a. 7). But because the Prophet Jehu declared to Josaphat, the wicked King of Juda (cf. 2 Chronicles 19:2 sqq.), that God had regard for his former merits, almost all other theologians consider it a "pious and probable opinion" that God, in granting the grace of conversion does not entirely disregard the merits lost by mortal sin, especially if the merits previously acquired surpass in number and weight the sins, which, perhaps, were due to weakness, and if those merits are not crushed, as it were, by a burden of iniquity (cf. Francisco Suárez, "De gratia", XII, 38). Prayer for future conversion from sin is indeed morally good and useful (cf. Psalm 70:9), because the disposition by which we sincerely wish to be freed as soon as possible from the state of enmity with God cannot but be pleasing to Him. Temporal blessings, such as health, freedom from extreme poverty, success in one's undertakings, seem to be objects of congruous merit only in so far as they are conducive to eternal salvation; for only on this hypothesis do they assume the character of actual graces (cf. Matthew 6:33). But, for obtaining temporal favours, prayer is more effective than meritorious works, provided that the granting of the petition be not against the designs of God or the true welfare of him who prays . The just man may merit de congruo for others (e.g., parents, relatives, and friends) whatever he is able to merit for himself: the grace of conversion, final perseverance, temporal blessings, nay even the very first prevenient grace (gratia prima prœveniens), (Summa Theol., I-II, Q. cxiv, a. 6) which he can in no wise merit for himself. St. Thomas gives as reason for this the intimate bond of friendship which sanctifying grace establishes between the just man and God. These effects are immeasurably strengthened by prayer for others; as it is beyond doubt that prayer plays an important part in the present economy of salvation. For further explanation see Francisco Suárez, "De gratia", XII, 38. Contrary to the opinion of a few theologians (e.g., Billuart), we hold that even a man in mortal sin, provided he co-operate with the first grace of conversion, is able to merit de congruo by his supernatural acts not only a series of graces which will lead to conversion, but finally justification itself; at all events it is certain that he may obtain these graces by prayer, made with the assistance of grace (cf. Psalm 50:9; Tobit 12:9; Daniel 4:24; Matthew 6:14).