What causes a hangover is that alcohol holds liquid and your body can't use it, so you get a headache because your brain doesn't get enough moisture, or something, so that's what you need to prevent.
So yeah, what Katie said, drinking a bunch of water before you go to bed should help, cause it moisturizes your body. Anyway, I usually don't suffer bad hangovers and I have rarely been sick of drinking alcohol, as in, throwing up, happened only twice in my life, last time like 7 years ago.. It's not that I drink very little, I can really drink quite a lot, but the point where I get really drunk to the level where I would get sick and get hangovers lies beyond the point where I just stop drinking because I've simply had enough and it doesn't taste good anymore.
You still with me..?