Bin bags are so useful. Especially when you need a poo in the longdrops, stick one around the seat, have a poo, when you're done just drop it in there.
Tent, sleeping bag, mat (it gets freezing without one). Chair is major important, they're like gold dust if you don't buy one down there early or bring one down and you will want to sit down.
Ducktape, it will fix ANYTHING and stuff WILL break.
Don't bring a pillow, bring a pillow case and stuff with clothes.
I spend £200-250 over the 5 days, including beer, food, fags, merchandise, wood etc. But £100-150 is doable if you can budget well.
Toilet roll (It will run out fast, or 12 year old tossers who've had a can of cider like to throw it in the bog)
Wet wipes, unless you want to shower. I tend to go for a swim at the leisure centre a couple of times over the weekend to wash the stink off, and it's lovely for a hangover.
Festival republic doesn't give out VIP passes except to pretty much the people playing the festival.
As far as camps go:
White: Familys, old couples, few stoners
Red: Stoners everywhere, very chilled
Orange: Stoners, drinkers, a bit lairy but not too bad, close to the arena and tescos
Yellow, Purple, Green: Depends which section (Usually more chilled as you get further from the arena) but usually very lairy, especially on sunday when many tents get burnt, I don't camp in these places but they are a laugh.
Brown: Crap, no one likes brown
If you've got an early bird ticket buy your booze down there (No glass allowed, buy 10p 2 litre water bottles, empty and use these to contain your spirits).
If you're down on the thursday, bring some down, tescos is usually done by thursday for all the offers and the big ripoff lorry full of beer is a ripoff.
Don't be tempted to take a can out of a crate as you're walking along, the crate will then fall apart when you're a good walk from your camp, I had to take my jeans off and use them as a bag last year.
Plasters and about twice as many socks as you think you'll need.
Don't padlock your tent, and avoid camping near roads. These are the tents that get robbed, don't leave anything decent in your tent, i've seen people have their wallet and phone nicked, ruins the weekend for them.
Headtorch for taking a dump in the middle of the night if the worst should happen.
And don't bring food, waste of space, bring more money to buy your food.
Oh yeah one last thing, don't take any amount of drugs you can't afford to lose. It's still illegal at reading and if they search you on entry they will confiscate it if found. A mate of mine lost his stash last year, £100 down the drain, I felt so bad for him.