The interior penance of the Christian can be expressed in many and various ways. Scripture and the Fathers insist above all on three forms, fasting, prayer, and almsgiving, which express conversion in relation to oneself, to God, and to others. Alongside the radical purification brought about by Baptism or martyrdom they cite as means of obtaining forgiveness of sins: effort at reconciliation with one's neighbor, tears of repentance, concern for the salvation of one's neighbor, the intercession of the saints, and the practice of charity "which covers a multitude of sins."
Source: Catechism of the Catholic Church (Vatican.va)