This is more of a potential cure and not prevention, but some research has shown caffeine and aspirin to be the best:

Once the source of the hangover headache was located, the researchers then set about figuring out how to get rid of the pesky things, using known headache blockers. Turns out the combination of caffeine and over-the-counter inflammatory drugs (i.e., NSAIDs – things like aspirin and ibuprofen) were best at blocking the head-pounding effects of the acetate.