Daily Renewal
One who wants to better himself is ipso facto not that to which he aspires. He lacks the virtues he wants to embody. His current modus operandi is to perform actions from which he would rather abstain and not to perform others that he wishes he would. The change he seeks is one of that modus operandi, his character.
The pursuit to change one’s character must be renewed every single day — routine performance of the actions he wishes to make default, writing down and committing to memory rationales for doing so, recitation of motivational tropes, conversations about virtue with like-minded friends, and, if one is lucky enough to have it, indulgence in culture that supports virtue and involvement in a community that makes it central.
As Pierre Hadot has shown well in both Philosophy as a Way of Life and What is Ancient Philosophy?, spiritual exercises were a central feature of Greek and Roman philosophy precisely because philosophy was for its early practitioners primarily an effort to attain wisdom and better oneself. Hence one would do well to read the likes of Plato, The Stoics, Epicurus, and Marcus Aurelius for examples of such exercises and to incorporate the best of them in his own life. The previous paragraph contains a few examples.