Depends on the type of hangover, really. One good tip is don't, and I mean don't, have coffee. That's a sure way to multiply it by 10. Sugary things are not a smart idea either. Water, lots of it. Vitamins. A glass of whiskey/cognac. Not beer though, you'll be off again.
OK the professional tip, but you may need to go down Boots for some of this. Pint glass, fill 30 % with fructose ( that's fruit sugar...do NOT use granulated as a substitute, totally different thing), add a pinch of salt and fill with water. Drink that, and do it again, maybe 3 x . That's your rehydrator. Then take a mega dose of Vitamin B 12, that'll get the sore braincells back in order. Chuck in some vitamin C as well, won't do any harm. But really, going down the pub is the best answer.