Mistake #1
Training on Empty
Low-carb diets are great if your idea of exercise is walking to work. But athletes need to fuel up, and that means carbs. "Endurance athletes perform best on a diet that's approximately 60 percent carbohydrate," says John Seifert, professor of health and human development at Montana State University and a leading sports-nutrition researcher. Most people don't eat enough carbs and don't eat enough before big workouts—causing them to exercise less intensely and therefore burn fewer calories.

The Fix: Experts say you should aim for 2.5 grams of carbs per pound of body weight every day. If you want to get a feel for this number, suffer through one day of strict carb counting and guesstimate from then on. Don't want to do math at lunch? Just make sure the biggest portion of every meal is made up of healthy carbs, like fruits, brightly colored vegetables, and whole grains. If you're going for a five-mile run, eat a carb-rich meal three hours beforehand. Example: 1 cup oatmeal with sliced banana, 8 oz low-fat yogurt, and 8 oz orange juice. Can't afford to wait three hours? Eat a bowl of cereal before leaving the house, and down a sports drink while you work out.