h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Miso soup is great for a hangover; it can be vegetarian (if you miss out the bonito flakes) has a mild, tasty flavour that's easy to swallow when hungover, and contains protein, water and salt.

If you want to tread the booze route, though, then it's hard to beat a bloody mary. My personal recipe goes like this:

vodka
tomato juice
tabasco
worcestershire sauce (must be Lea and Perrins)
horseradish
ice

stir as vigorously as you can manage.

drizzle a little sweet sherry over the top of the drink; the flavour of tomatoes is brought out well by the sugar in the sherry.
Put a stick of celery in it.

Drink.

Vodka and tonic is also very good.

Cheesy doritos always work. I have absolutely no idea why. This does presume you've either got some in or you're in a fit state to go down to the shops to get some though.