If you are going to a music/art festival, you need to take a few things:

tent (and don't forget the tent-pegs!)
warm sleeping bag
sleeping mat (the ground can be very uncomfortable and cheap tents offer no protection from stones)
baby wipes (you may not be washing for 3 days or more so these are a MUST)
deodorant (what, are you a stinker?!)
any medication you take regularly and keep it with you at ALL times
a mirror (well, you want to look good, right?)
toilet paper (need you ask??!)
condoms? - N/A for me as I'm a gay woman but I thought I should mention it anyway...
ear-plugs (seriously, bongos do not make for a good sleep and you may be needing to sleep in the day as your pattern gets messed up at festivals and these days, you get screaming babies on-site too)
paracetamol (for general pain)
sanitary towels (just in case, girls - they sell them at extortionate prices on the festival site)
walking boots or wellington boots (you will be walking miles over the course of the weekend - wear proper mud-proof and waterproof footwear!)
spare clothes (expect a variety of substances to get onto you at the weekend - you'll need to change)
waterproof anorak (a MUST)
waterproof trousers (a MUST)
sun protection (a MUST)
woolly hat (for nightime)
thermals (you'll thank me when you are the only one not freezing in the night when temperatures drop)
peaked cap (the sun is powerful)
a scarf (as above and it blocks wind and doubles as a mask for coping with chemical loos on day 3)
decent socks (avoid blisters!)
fancy dress attire (do it!)

NB) Don't take your expensive phone, camera or mp3 player! Take a cheap festie phone, a disposable camera and you won't need an mp3 player - you'll have live bands almost 24-7!

Off you go then and don't do anything I wouldn't do. May the force be with you...