Don't go out and binge on pasta the night before. Just eat a standard portion of a carb-based meal you're familiar with. Then, the morning of the race, have an early breakfast. "Get up two to three hours before the start and eat something high in carbohydrates and low in fat and fiber," says pro triathlete and nutritionist Pip Taylor. Toast, bananas, and sports drinks are staples; energy bars also work.
Throughout the bike and run, you want to deliver a steady stream of calories and water to your working muscles—up to 60 grams of carbs and 20 ounces of fluid per hour. On the run, drink stations will be set up every mile, but bring your own gels—wash one down with 8 or 10 ounces of water every half-hour. (And sample different products in training so you know what will work best for your system on race day.)
Bounce back quickly by downing a liquid meal supplement with a mix of carbohydrates and lean protein, like Endurox R4 ($18 for six; accelsport.com) or Muscle Milk ($3; cytosport.com) within 40 minutes of finishing. Keep one on hand for after the race. (Following hard workouts, simple combos like cereal and milk or chicken and rice work just as well, says Taylor).