I used a tutor to study for the SAT in high school, and he gave me some good eating tips. I've used it ever since, and I've done pretty good on them all. Here were the tips:
The day before, drink lots of water, eat lots of carbs the night before.
The morning of, drink two tall glasses orange juice, 3 eggs with lots of cheese and 2 pieces of toast.
For me, it works, and I always feel great when taking the test. I always avoid cafeine. Yes, it makes me feel alert, but I ALWAYS miss the little things, or fall for the trick answer when I do that. I avoid it. Water is best.