Eat carbs as your lunch or dinner, as I'm assuming that long of a session or tourney you would eat some time. Also don;t drink energy drinks until about the FT if you have one, I read an article in bluff, since energy drinks make you crash after about an hour or so.
Well, the best way to cope with fatigue is to take breaks, but not just 5 minute ones. I would suggest going on a walk outside for thirty minutes or so or even taking a power nap. You need to be focused playing good poker, and if you find yourself tired, it's smarter to rest and play well for less time than to play worse for more time.