The Need to have's

Your ticket!
strong and sturdy but comfortable (!) shoes. You will walk quite a lot and blisters are a festival mood killer.
A dust mask (those you can buy at a hardware store) The opposite of heavy mud is heavy dust, and that's just as bad.

Clothes:
Bring some shorts and tshirt.
some pant and long-sleeved t-shirts in case you get sunburnt.
A warm sweater for nighttime.
A raincoat of some kind and water proof boots.
Don't bring too much clothes as chances are good that you will wear the same stuff several days in a row.
Don't bring anything that you will be sad about getting ruined. Ofcourse you will get a lot of streetrespect when you wear that 1982 - "U2 on Tour" t-shirt but you will be sad when someone rips it because he trips and falls into you.
plastic money (EC or Visa will do) and some cash
Gaffa Tape
Tent. Must be something that you will not regret if it gets thrashed or you decide to leave it.
Sleeping bag. Again something that you won't get sad if it gets vomit on.
Some kind of matress. Inflatable is best but is heavy to carry.

Hygiene: A toothbrush, shampoo, two towels, hairproduct if you use it. Keep it basic. Don't worry too much about looking sharp because you will loose that edge after a few hours in the dust and mud anyway. DONT BRING MIRRORS,
Condoms, if you intend to have sex. In that case you might want to be a little more picky about the hygiene part, as well.

The Nice to have's

A hidden money bag (those things you wear around your bag or inside your pants)
A Camp cd-player
Batteries
CDs for the camp player
Tempos (for non-germans: Kleenex)
Bathing Sandals
Earplugs
Camera
Plastic bag for dirty clothes
Brausepulver
Sun Lotion
Medication: Although this largely depends on you, pain killers, coal tablets and antacids will probably be needed by at least one person in your camp.
Hard plastic cups.
Liquor and Soda
Beer
Some cheap folding chairs. You get really tired from sitting on the ground or an empty beer ate all the time.
A (cheap) multi tool can be very useful