The Wookiee
Back From The Dead
Berroca
Isotonic sports drinks. If you're dehydrated, which you inevitably are following excess alcohol consumption, then your salt levels get low, same as they do when you sweat lots following exercise. Drinking loads of water will only dilute your salt levels further, which is why you get a headache. Isotonic stuff like Lucozade Sport (or remedies like Dioralyte as others have said) are spot on here, because they replace the salts you've lost, at the same time as getting more fluids into you.
I also recommend a monstrous carbohydrate overload. I've no idea if this part actually has any medical benefit, but it always makes me feel better.
This is very much "do as I say, not as I do", as I'm rarely organised enough to have Lucozade Sport in stock at home in the event of one over the eight...