1. Know yourself! Know how much you can handle, and stick to it.
2. WATER. Like Delicious Surprise said, it'd be good to alternate alcoholic drinks with water. A whole glass before bed, and WHEN you wake up in the middle of the night to pee (not if lol), another glass of water.
3. FOOD. Eat before you drink, eat while you're drinking, eat afterwards. Unless you feel sick, then don't force it... but eating while you drink really helps.
4. Tylenol is harsh on the liver, particularly if you've been drinking. Don't take it. Apsirin can also cause stomach aches when you're hungover.
The only hangovers I've gotten before were from wine, which was weird because I wasn't drunk either time (I've never been drunk, actually. I've hit that almost-but-not-quite-t here-yet point and didn't like how it made me feel, so I stick to just a drink or two). For some reason, wine hits my head more than any other drink. Water and crackers solved it, though.
Also, watch Season 6 episode 16 of MythBusters. They go through hilarious hangover "cures".