I had a really specific thing going, because my body can't tolerate caffeine - it feels more like poison than stimulant. I drank a glass of double strength emergen-C in the morning with my multivitamins, had a decaf soda and banana for short breaks (for potassium and sugar/fizzy wakefulness respectively) and then during the long break, I had a meal rich in complex carbs, green vegetables, and sodium, supplemented with almonds and coconut water fortified with a hearty scoop of pure dextrose and Vit D.
Remember, your brain needs sodium and potassium to run, and your muscles need calcium (from almonds mostly). Simple carbs will make you sleepy, so stick to complex carbs, and the dextrose halfway through as it is the fastest possible energy source. Hydration is important, so stay away from diuretics like coffee/tea, and whatever you do, DON'T TAKE 5 HOUR ENERGY. Repeat, DON'T TAKE 5 HOUR ENERGY. Ephedrine, the stuff in it, is a powerful sympathomimetic that will make it impossible to sit in your chair, raise your heart rate, and will make your blood feel like it's rushing - it's side effects are restlessness and confusion. You don't need a fight or flight response during your exam, you want to calm down. It's meant for physical, not mental exertion.
Oh yeah, and that turkey sandwich you were planning to have for lunch? Don't. Tryptophan is a powerful soporific and is commonly associated with turkey, though it's found in most meats. It is also metabolized from dairy.
TL;DR Use some of the B/B training to figure out what your body should have and make sure you have all of it.
