"One should be careful to perform a small mitzvah just as he would perform a big mitzvah, for one does not know the reward allotted for mitzvos" (Pirkei Avos 2,1). Similarly, Chazal tell us that the pasuk in Tehillim, "Great is the reward reserved for those that fear you", refers to the reward allocated for the performance of small mitzvos. Likewise, Rashi in the beginning of this week's parsha explains, "V'haya eikev tishma'oon", if you perform the small mitzvos that one treads upon with his heal (eikev); Hashem will safeguard His covenant and kindness that He swore to your forefathers.
Someone who is bent on changing the world thinks about founding international organizations promoting peace or charity. Similarly, the thoughts of one who simply wants to improve his character revolve around great acts of kindness or remarkable accomplishments. However, the aforementioned statements of Chazal contradict such ideas. Rav Wolbe (Alei Shur vol. II pgs. 189,190) adds, that just as the world is made up of indiscernible atoms and our bodies are made up of microscopic cells, so too, our spiritual makeup is defined by our small actions; for good or for bad.
The potency of small actions is comparable to a pill where the active ingredient makes up merely one or two percent of the entire dosage, and a greater quantity would do more harm than good. Likewise, small actions do not arouse our instinctive feelings of opposition. One who takes upon himself a grand resolution that causes him to feel pressured, will slowly but surely feel a desire to rebel.
Rav Wolbe related that after the Yom Kippur War he flew to Egypt. When the plane entered Egyptian air space, he noticed that they were flying extremely low, just a few meters above the ground, and he asked if they were experiencing engine trouble. He was told that because they had entered Egyptian air space, they were flying beneath the height detectable by radar, lest they be noticed. Rav Wolbe applied this idea to spiritual resolutions. Our instinctive feelings of rebellion only detect grandiose actions and resolutions, while the small resolutions go unnoticed.
When one focuses on changing a specific character trait, the most practical solution is to work with small steps. One small action done continuously for numerous days does not overwhelm a person, while it still has the ability to change him for the better.