Now this is a topic I approve!
Well, as people mentioned above, drink water, a LOT of it. Usually I take a bit of water at every toilet visit when drinking, and trust me - that's a lot.
For me drinking water and alcohol is not a problem at all, but a lot complains that the water only gets them more ... what's the name for it, in need of puking.
When it comes to the day after, please do not abuse various pills (such as those for fever, headache and such). What you ultimately do is to make the body resistant to the medicines, and when you one day TRUELLY need them, they won't be as effective.
Suck it up, endure the pain, or - continue drinking (as someone said earlier too) and everything is ok.
For me I eat a lot of food, drink a lot of water and everything is cool... most times.. but some times it's a hangover from hell, and I am chained to the bed beeing misserable lol.
Ultimately - smoke a join and get high instead, or that's at least what I have heared. No hangover then!