This is obvious. Don't buy one you can't put up. If you buy your tent in the Blacks sale, 80% of the campsite will have the same one, so pack a flag or bunting to make it easier to find at 3am. Getting into bed and cuddling up with the wrong person is not the way to make new friends.
Torch: and batteries. Or a wind-up torch if you're somebody who loses 
batteries (I am). Head torches are also useful for navigating portaloos 
at night - just for heaven's sake don't look down.
Sleeping bag: If you can sleep anywhere, this is all you will need. If 
you're high maintenance like me, you will also need an airbed, foot 
pump, extra duvet and fifty pillows woven from the flaxen hair of mermaids.
Camping chair: You'll laugh, until everyone else in your group has one 
and you have to sit on the ground. And then you will cry. 
iPod dock/iPod/speakers/relevant batteries: You'll need tunes for when 
the site shuts down early on Sunday and you still want to party. Don't 
bring a ghetto blaster though, or your neighbours will piss in your tent
 while you're out. Be respectful of those around you.
Waterproofs: Essential. Even if it doesn't rain, there will be mud 
somewhere, and you will stand in it. Wellies, mac, waterproof trousers.
Binliners: These will function as waterproofs when all your waterproofs 
are muddy. They will also cover your stuff when you're queuing to get 
onsite in the pouring rain, keep your feet dry when your boots leak, 
hold all your campsite empties and give you calm-down chats when 
somebody spikes your drink with acid. Binliners are the answer to 
everything.
Wet-Wipes: A shower in a packet. Necessary.
Dry shampoo:  See above. You will plan to visit the onsite showers, but 
unless it's scorching hot you probably won't do it. If you do, odds are you'll get trenchfoot. Plan to go four days without washing - 
then if you manage it, it's a bonus. 
Water: Take one of those huge fold-down camping cartons and you won't waste the festival permanently in the queue for the tap.
Loo roll/tissue handypacks: An hour after the gates open, all the toilet
 roll in all the toilets will simultaneously evaporate. Be prepared.
Cash: Take more than you need if there's no ATM on site. If there is, 
expect to be queueing for it all afternoon while all your friends (who 
brought more cash than they needed) are watching Mr Scruff.
Tickets: Don't be an idiot. Check if you need a car-park pass too, a lot
 of festivals charge for this now. You can sometimes buy in advance 
for cheaper, which also saves more queueing when you get there.
Suncream: Because you just never know.
The Rest:
Phone: Keep it switched off and for emergencies only. Have an agreed 
meeting point with friends instead and use your 
actual watch - just like you did before 1998. If you tweet about it, you weren't there.
Clothes:  Spare socks are always 
good, warm layers too. Don't pack a maxi dress unless you want a hem 
bedecked with manky loo paper and mud. You're not at Coachella (unless you actually are). And 
don't pack anything -particularly footwear- that you'd be devastated to 
lose or damage. As for the rest, anything goes. Be brave if there's a 
fancy dress theme, you may find your festival self is not like anybody 
you've ever met before, so go with it. Unless "it" involves wearing one 
of those fucking jester hats with bells on.
Booze: Always check the festival's rules and restrictions. If you're going to 
be naughty and take in more than your allocation then you'll need to be very 
MacGyver about it. It 
is possible to sneak in extra cans of beer... 
providing you sew them into your pantomime horse costume.
Hand sanitiser gel:  Also known as "sanity".
Tent mallet: Best way to make friends with your neighbours when you're pitching. Nobody else ever thinks to bring one.
Food: Nothing immediatley perishable. Your tent will smell cheesy enough
 already without putting ACTUAL cheese in it. Even if you're planning
 to eat at the facilities onsite it's worth having a few snacks in the tent for those 
mornings where you're not quite sure if your blood sugar levels will be 
sufficient to get your legs to the nearest coffee stand.
Rehydration Salts: Ideally taken in tandem with Alkaseltzer XS and a 
Berocca - BOOM! - fresh as a daisy. It will taste like witch's vomit, but give it 15 minutes to settle and the world will stop looking like a negative of itself.
Safety pins: Because you will always need safety pins. Hangbags can fall apart, tents can rip and monkey's tails can fall off.
Gaffer tape: Because you will  always need gaffer tape, even if it's just to stick a Hitler tache on your best mate while they're asleep.
and finally... 
Glowsticks: Because you will always, ALWAYS need glowsticks.
 All pictures taken at the Secret Garden Party 2008-2011