A TV show called Brainiac researched this. The only effective way to reduce the effect of a hangover is to drink water, not only during the partying, but some time before. I'm sure that it does not need to be an excessive amount.

On a side note, you ought to drink water anyway because imbibing alcohol can seriously dehydrate your body. In fact, my tongue is currently plastered to the roof of my mouth for that exact reason. I guess I should practice what I preach.

Also somewhat related: Drink water on normal days as well. My friend once collapsed from dehydration and had to be hospitalized. The doctors put salt in his ******* veins to help retain moisture. He describes it as "the most painful experience of my life" and drinks water almost obsessively now.