Again, the greatest Doctor of the Church explains how one gets merit, but may I add that only those who are purged of sin can really merit. The Church, for example, tells us that we must be free of the desire to sin even venially in order to obtain a plenary indulgence. Here is Aquinas on how to gain merit. Merit must involve charity or love of neighbor. It must involved something difficult and not easy, like extra prayers and extra devotions, as well as love.
And, we merit reward for loving for the sake of Christ and not for the sake of the person involved.
As we may gather from what has been stated above (Article 1), human acts have the nature of merit from two causes: first and chiefly from the Divine ordination, inasmuch as acts are said to merit that good to which man is divinely ordained. Secondly, on the part of free-will, inasmuch as man, more than other creatures, has the power of voluntary acts by acting by himself. And in both these ways does merit chiefly rest with charity. For we must bear in mind that everlasting life consists in the enjoyment of God. Now the human mind's movement to the fruition of the Divine good is the proper act of charity, whereby all the acts of the other virtues are ordained to this end, since all the other virtues are commanded by charity. Hence the merit of life everlasting pertains first to charity, and secondly, to the other virtues, inasmuch as their acts are commanded by charity. So, likewise, is it manifest that what we do out of love we do most willingly. Hence, even inasmuch as merit depends on voluntariness, merit is chiefly attributed to charity.
Reply to Objection 1. Charity, inasmuch as it has the last end for object, moves the other virtues to act. For the habit to which the end pertains always commands the habits to which the means pertain, as was said above (Question 9, Article 1).
Reply to Objection 2. A work can be toilsome and difficult in two ways: first, from the greatness of the work, and thus the greatness of the work pertains to the increase of merit; and thus charity does not lessen the toil--rather, it makes us undertake the greatest toils, "for it does great things, if it exists," as Gregory says (Hom. in Evang. xxx). Secondly, from the defect of the operator; for what is not done with a ready will is hard and difficult to all of us, and this toil lessens merit and is removed by charity.
Reply to Objection 3. The act of faith is not meritorious unless "faith . . . worketh by charity" (Galatians 5:6). So, too, the acts of patience and fortitude are not meritorious unless a man does them out of charity, according to 1 Corinthians 13:3: "If I should deliver my body to be burned, and have not charity, it profiteth me nothing."