The obvious thing is 3-4 pints of water before bed, but life isn't always like that. Failing that, when you wake up needing the 6am piss, drag your sorry arse to the kitchen and have a full pint of Ribena, two co proxomol (500/30s if you can get them), a couple of rennie, then take another pint of Ribena to bed and have it before dropping off again.
You'll come up smiling a few hours later. Don't have a full English as the fat will knock you on your arse again. Have loads of coffee and maybe a nice bacon butty on fresh, cheap white bread. Keep the liquids going in and maybe a couple more co proxomol. If the situation allows don't rule out a joint or two.
The above will get you through the worst hangovers. The only exception are those where you are so bad that at the 6am piss break you vomit up the drink and tablet. Then you're right in the shit.