Condoms, take them: Filthy field sex is bad enough as it is, and these can be quite hard to come by on site - so be clever and pick some up before end. If you're too drunk to use them, you can always inflate them and throw them around the main stage on the last night, like a tool.
Orange juice: If you're suffering a comedown, Vitamin C helps. Science!
Share your spirits: You'll probably want to guard that 16-pack of cider with your life as it isn't going to last forever, but if you've managed to smuggle in a bottle of vodka in some secret pocket or Godforsaken orifice, then get into the festival spirit and share it with your buddies. You wouldn't down a bottle of rum to yourself at home, and it's only going to hurt all the more if you have to sleep outside.
Don't eat spicy or suspicious food: A festival toilet is no place to experience the runs.
Take an airbed: They're surprisingly cheap, easy to carry and inflate - and make a world of difference. Turn four days of hard-partying interspersed with sleeping on a cold, hard floor into feeling like an absolute KING/QUEEN. Just look how HAPPY these guys are.
Deal with muddy and wet clothes: Don't get smashed and pass out in your muddy boots and soaked clothes, you'll stink and die. Leave them in the porch of your tent.
Mud-wrestling: Just don't
EAT: Don't get too caught up in the constant party. Eat something hearty and substantial before each big session.
CRISPS: The salt in crisps replaces that lost from drinking. More science for you, there.
WATER: Drink it hourly, it will keep you going.
Don't be fooled by caffeine: It may seem like a cracking pick-me up, but caffeine is only a temporary cure and will dehydrate you in the long-run, so drink other fluids with it.
Don't mix booze and drugs: That's a general rule of life
Shade, find some: You may want to top up your tan, but do so in moderation and don't spend all day raving at the mainstage in the scorching heat, drunk and oblivious to the fact that you're burning or developing heatstroke.
A festival hangover is like any other - but outside, in fancy dress and much less comfortable. As per usual, drink a shitload of water before bed - but be prepared to make an epic toilet dash in the morning.
Get some 5 HTP: Not that we're condoning the taking of any naughty substances, but if you absolutely must, then pick up some 5 HTP from a chemist or health food shop before. It will restore you to your full self in no time, like a mushroom to Mario.
Cure the post-festival blues: Don't go straight back to work without taking proper care of yourself. You'll have a nervous breakdown and end up weeping the foetal position. Drink loads of water, get a decent night's sleep or two, and apparently if you drink milk thistle and dandelion complex for a few weeks and eat loads of green veg and yoghurt it sorts out any digestive problems you may have.