Modern medicine will have you believe that a hangover is primarily caused by hypoglycemia, dehydration, acetaldehyde intoxication and vitamin B12 deficiency (thanks Wikipedia) but if you believe that guff, you'll believe anything. After exhaustive studies on the subject, Captain Drib Drab and I have discovered that the hangover is really the evil work of a sadistic rhino who exists in the cosmos and comes down to visit after you've had a hairy night on the sauce. His best pal is an equally sadistic eagle, and they spend all day playing chess and devising new and elaborate ways of making life more vile and intolerable than it already is.
A fairly effective method for keeping them at bay is to stay on the move, or better still, get out of town for the day. The rhino is a slow moving beast, so if you go somewhere new it can take him a while to track you down. You can also try wearing a disguise. I recommend a fake beard and glasses, but you can use whatever takes your fancy. Unfortunately, he has the eagle to help him out who obviously has the advantage of flight, so it's also important to watch the skies.
I suppose the only authentic way of avoiding a hangover is to abstain from booze altogether, but even I haven't got a mind so sick as to recommend something that stupid. Drink responsibly, doinks.