Of course, the best way to deal with a hangover is not to get one in the first place. However, since that doesn't seem to be an option , the next best thing is to do what kellyuk said: drink as much water as you can manage while you are also drinking booze. It's easier to deal with a hangover in advance by minimizing the damage. Go for a 2:1 ratio if you can. I use the same glass and fill it up twice with water for every one alcoholic drink I have.

Emily