No matter how fit you are — or how badly you want to lose those last 10 pounds — working out every single day without a break will break you. "Taking days off is vital," says William J. Kraemer, a professor of kinesiology at the Human Performance Laboratory at the University of Connecticut, in Storrs. "Rest allows tissue to recover and helps you to refresh yourself mentally. A program without planned rest each week is doomed to failure." To avoid overtraining, Kraemer suggests taking a day or two off each week and up to a week off after a strenuous event, like a marathon.