It is always better to pack more and be prepared than to pack less and be sorry. This is my general list of what to bring to a Festival
Tent - Something sturdy and big enough to last the weekend, always get a size bigger as a two man tent won't be large enough for two people and their stuff.
Ticket - Obvious
Clothes - dependent on fashion taste but make sure to bring some warm clothes, a coat and wellies.
Wallet - Including I.D. and bank cards, never nice to run out of money or get refused at a bar. Keep on person at all times and look out for pickpockets
Smokes - If you smoke be sure to bring enough to last you the weekend.
Toiletries - a weekend without showering can be pretty horrid. Bring baby wipes, toothpaste, deodorant and some aftershave. Also condoms, never know what might happen and better to play it safe.
Camping Chair - Essential if you want to sit and relax in the mornings while getting pissed.
Duct tape - Tents rip easily and duct tape can easily fix that and help repair broken chairs.
Bin bags - to clear up the mess of course.
Food - Simple easy food like chocolate bars, sandwiches and crisps.
Medicine - paracetamol to help with the hangover, pro-plus to keep you going and any medication you might need like hayhever pills.
Drink - Obviously bring your alcohol but don't forget water to help keep hydrated and soft drinkers for a mixer.