The best hangover preventions are food and water
(or other non-alcoholic drinks). Take in plenty of these when you
drink, and you'll do fine. Myself I usually alternate beer and water
when am drinking; and can get stupid drunk, and never have a hangover.
(Me, I never get drunk in the first place; I keep my beer consumption
to 2-3 a night, partly because I don't particularly like getting
blasted, and partly because I can't stand cheap beer and can't afford
enough of the good stuff).
And to whoever said soda is a diuretic; it's true, caffeine is a
diuretic, but soda doesn't contain nearly enough to even counteract its
own water content, let alone make you more dehydrated. I read somewhere
that 3 cups of soda is just as hydrating as 2 cups of water —
definitely less efficient, but it will help. Plus, caffeine does have a
(mild) anticoagulant effect, which can help relieve pain caused by
inflammation.
Being a heavy consumer of craft beers I find
myself with relatively frequent hangovers (I'm at the point where I
don't even have to be drunk to get hit with 'em anymore) and have found
the ultimate cure. The Vietnamese soup known as pho. No matter how much
my head is throbbing—no matter how much I wanna vomit with every
movement—as soon as I begin getting a bit of pho down, I'm well on the
way back to being bright-eyed and bushy-tailed.
Next time, look up the nearest pho house and start slurping. It's magical.