By 'simple', I mean the three basics: shorts, t-shirt and wellies. Pack as many spare changes of underwear and thick socks as you can; also, a cagoule or waterproof poncho on hand can prove to be a lifesaver.
Don't wear or even bring jewellery or make-up (except for an old watch and a splash of rave-paint), as these aren't essential for trekking around muddy fields and getting lost in mosh-pits.
Wellies are essential, I once saw a poor child calf-height in mud, wearing Crocs! Walking boots and trainers don't quite cut it either.
Wet-wipes: very important
Never use festival showers. If you need to get clean, stock up with as many packets of wet-wipes as you can. It will prevent you from becoming and stinky/muddy as your peers at the end of the weekend.
Always have some spare cash hidden away
Festival prices are pretty extortionate, and the available cash machines will have a charge to withdraw. Pack up cereal bars, water and chocolate (or anything else you know you might fancy), and ensure you have enough cash to buy the things you want (official t-shirts, burgers and beer). Head out to the campsite supermarket in the evening/night time if you fancy cooking your own food (the supermarkets will be less busy during the evenings).
No valuables unattended in the tent
Leave things like spare clothes in the tent during the day time; take your phone, wallet, camera, family jewels etc with you when you head out to see the bands.
Enjoy yourself, stay safe, and remember: festival season comes only once a year!