The New York Times reports that while exercising willpower in one area of your life will reduce it in others, practicing willpower improves your capacity for keeping yourself in check overall.
The brain has a limited capacity for self-regulation, so exerting willpower in one area often leads to backsliding in others. The good news, however, is that practice increases willpower capacity, so that in the long run, buying less now may improve our ability to achieve future goals—like losing those 10 pounds we gained when we weren't out shopping. This means that you should pursue one or two goals at a time, and let the other stuff go. Then, when you achieve them, you can move onto the next accomplishment.
Tighten Your Belt, Strengthen Your Mind [NYT via Anne Truitt Zelenka]