"Your body needs to be refueled every four hours or so for your energy to stay at an even level," says Miriam E. Nelson, Ph.D., author of Strong Women Stay Young, $14, www.amazon.com. Three small meals a day with substantial snacks in between should do it — just be sure to get the right balance of protein, carbohydrates, and fat.

To avoid sudden energy crashes, eat a little protein (a small serving of lean meat, beans, or milk) at each meal. Protein is metabolized more slowly than carbohydrates and thus provides more long-lasting fuel. The rest of your diet should consist of complex carbohydrates, such as fruits, vegetables, and whole grains. "Complex carbohydrates take longer to digest than simple carbohydrates like sugar and help release energy when you need it," explains Felicia Busch, author of The New Nutrition, $17, www.amazon.com. And beware of cutting too much fat from your diet. Don't gasp: Fat is an energy source that can sustain you for hours.