Willpower gets depleted with use. When the tank is empty, give yourself a rest before tackling a new self-control challenge.
You can speed up your recovery by doing something to lift your spirits, rewarding yourself for good behavior, or by just thinking about someone you know with lots of self-control.
Your willpower muscle will grow with regular exercise. Before taking on a goal that requires lots of willpower (e.g., quitting smoking, radically changing your diet), start by strengthening your muscle with regular, less strenuous workouts. Add a few willpower challenges to your day (e.g., making your bed, sitting up straight, taking the stairs instead of the elevator) and build from there.