water, water, and more water.... rekcaSxT was spot on, hangovers are the result of dehydration.
The only thing that I would add is that I drink a glass of water between every liquor drink. This serves a few functions, it helps keep me hydrated, it slows down my alcohol consumption, water is cheaper than a mixed drink (if they charge at all), and I can walk around with a glass of water and still have a drink in my hand... I feel aykward at a bar without a drink in my hand.
LF