Drink water, take painkillers and your medication of choice for an upset stomach, and eat whatever you can get in you. I've heard arguments for both shoving an entire rasher of bacon in your face and also only eating dry toast or whatever; as far as I'm concerned you should go for whichever you think you can stomach. Sleeping it off is good too.

Best way to deal with it is to drink a fuckton of water along with your booze, though, then you hopefully don't have to deal with the actual hangover at all.