WATER. I know it's already been said, but you can't get enough of this stuff (ok you can, but hyper-hydration is pretty extreme). I never used to drink much water but in the last year or so I've upped my intake to at least 3L a day, sometimes 5 or more if it's hot and I'm being really active. I will fill really bad (ill almost) if I don't have a couple of litres a day. Yes you do tend to piss a lot, but that's the idea, get all the crap you don't need out of your system and keep it well fueled.
Anyway, that's my two cents, I'm gonna shut up now before I say something really misinformed...