According to Osho, willpower grows only by exercising it: make small, doable resolves and unfailingly complete them. Each kept vow strengthens self-trust; each broken one weakens resolve and self-respect. Avoid grand pledges; practice steady, simple disciplines (like fixing your gaze for an hour) and persist. There is no shortcut—do exactly what you wish to cultivate, consistently, and it will develop.
You build willpower by making tiny promises to yourself and keeping them, again and again.