I've been using the Chaser brand hangover prevention pills for years. I just recently switched to the Sobr-K brand of "Hangover Stoppers" since they are much cheaper and work just as well if not better.
They're activated carbon (a.k.a. charcoal) pills that absorb the toxins in liquor. They really do work. One of the few times that I forget to take them and went out drinking, I was hurting the next day.
http://www.hangoverstoppers.com/