beer+wine=
A Little Drink to Keep You Awake

Something sugary, but only if the time control is less than 45 mins. (That's when sugar has the best effects on your brain. ... I think ...)
Water is always good until you have to go to the bathroom in the middle of the game.
When you play long games of chess, what kind of drink helps you stay awake the most?
Smoothies? Soda? Water? Juice? etc.