The Secret to Strong Willpower?

"When you use self-control, it gets a little more depleted," says Kathleen Martin Ginis, PhD, professor of kinesiology at McMaster University, in Hamilton, Ontario. And once you're running low, you're less likely to exercise, eat well, or for that matter, pass up a sale on cashmere cardigans.

Last September Martin Ginis published a study in which 61 men and women were asked to exercise; half of the subjects were then given a willpower-draining task. Afterward, those who hadn't performed the task worked out with greater intensity than those who had.

But research suggests that if the exercisers had continued to perform the task over time, their willpower would ultimately have improved. "It's like a muscle that gets stronger with use," says Martin Ginis. In another experiment at Macquarie University in Sydney, Australia, people who were asked to follow a money management program eventually began eating healthier diets, drinking less alcohol, and smoking fewer cigarettes. The program, it turns out, increased the subjects' general willpower reserves. You can boost yours, too, with a few simple exercises: Try to avoid swearing or saying um ; focus on sitting up straight; or resist checking your e-mail every few minutes. "It may sound silly," says Martin Ginis, "but it works."

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