The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) begins the seventh chapter of Hilchot Teshuva (listen to audio for precise citation) by emphasizing the importance and value of Teshuva (repentance). Having established in the previous chapter the philosophical basis of the doctrine of free will, that God does not interfere with man's decision to act properly or sinfully, Maimonides now writes that one must exercise his free will by repenting from his misdeeds. A person has the ability to refine his conduct and thereby earn a share in the World to Come, and it therefore behooves each and every individual to perform Teshuva and correct his flaws.
In the subsequent passage (Halacha 2; listen to audio for precise citation), the Rambam adds that one should make a point of repenting each and every day of his life. Nobody knows how long he will live; not every person is blessed with longevity. As such, it is unwise to delay Teshuva and think to oneself that he will have time to repent when he reaches old age. Death could come upon us at any moment, and we must therefore ensure to repent immediately upon committing a transgression, and to clear our record each day. The Rambam cites in this context the verse from the Book of Kohelet (9:8), "At all times your garments shall be white." One must see to it at all times that he is "white," pure of sin, as any moment could prove to be his last, Heaven forbid.
In Halacha 3 (listen to audio for precise citation), the Rambam teaches that one must repent not only for the wrongful acts for he committed, but also for negative character traits. The Rambam lists here numerous tendencies for which one must repent, including anger, competitiveness, envy, lust for money or fame, and overindulgence in food. Even if these tendencies have not resulted in particular misdeeds, they nevertheless require one to repent. In fact, the Rambam adds, Teshuva for negative tendencies is generally more difficult than repenting for specific acts of sin. Unlike isolated acts, negative tendencies become second nature and part of one's personality, thus making it particularly difficult to overcome them. One must therefore invest considerable effort to refine his character and rid himself of these negative qualities.
Moreover, the masters of "Musar" have taught that repenting for negative character traits often holds the key to successful Teshuva for specific acts. Virtually all acts of sin, they observe, result from inherent flaws in one's character. Thus, by cleansing one's personality of negative tendencies, he can help ensure to avoid misconduct. Hence, perfecting one's character by ridding himself of the negative qualities listed above constitutes the cornerstone of the Teshuva process and must therefore be given primary focus and attention as part of our efforts to fully repent.