Before Practice: At least a few hours before eat a high-carb meals and snacks. I like to eat quesadillas with chicken, a PB & J, or spaghetti and meatballs.
During Practice: Drink a lot of fluid and eat high-carb snacks. (sports drinks, power bars, power bar or gatorade gel snacks). With a well nourished body, practices can be longer and harder; giving better results.
After Practice: Replenish the store of glycogen (carbs) in your body with high-carb food and drinks.
At a Meet: 15 minutes before a race have one bite of an energy bar with a lot of protein and during a little bit of Powerade for the extra protein and electrolytes before the race.
I personally believe that this is why I get fatigued rather fast in practice. I need to nourish my body better in order for it to keep up with the rigorous exercise I'm doing. I think this will definitely help me from "hitting the wall" at practice and at a meet.