I find the appropriate drink is dependent not so much on what you're writing, but on how you're writing. For those wonderfully lucid times when everything's just flowing, a nice red wine is tasty, inspiring and not too harsh. For the really gnarly bits where you really need to keep stopping, thinking well ahead, and playing around with phrases at length before you set them down, a decent whisky is a wonderful thing, not least because you can play around with it in the glass and enjoy the fact that Whisky Is Pretty (this is particularly satisfying and beneficial if you have a
Glencairn glass, which if you're drinking whisky, you really should - and let's face it, you really should be drinking whisky.) Somewhere between the two, where it's taking some work but you're not pulling your hair over every word, I like green tea, not least because the preparation gives you time to step back. (There's no reason not to drink black tea instead, of course, but I just get bits of soggy biscuit in my keyboard if I try that.)
Redrafting is a nasty bastard and in my experience responds only to large volumes of strong, black coffee and plenty of water.
I'd also second wombat's point on the booze - whatever you're drinking, don't over-addle yourself. For every brilliant flash of drunken inspiration there'll be a thousand literary equivalents of pissing in your waste-paper basket.
Finally, and most importantly: if you find yourself struggling it's probably not because you've got the wrong drink, so try mixing it up elsewhere, and don't feel you should be writing in a freezing garret with a pair of fingerless gloves and a bottle of booze if you'd rather do it on a park swing with a mango smoothie.