I've gone through this entire thread and grabbed most items. I'll sort them into a list of essentials and non-essentials. Obviously you can change the list for yourself but this covers me to a T. You guys rock!!!
Essentials:
Water (About 5 litres per person per day IMO)
Lipbalm
Warm cloths, cool cloths, and a change of cloths, socks (lots!)
Sunglasses
Spare socks/jocks etc
Toilet Paper
Torch
Hat/beenie
Blanket (or sleeping bag/doona) to sit on & pillows.
Tent - A hammer & tent pegs
toothbrushes/paste/washing stuff
Melways/a decent map of surrounding area
garbage bags
first aid kit
Blistex. Bandaids. Baby wipes.
Earplugs
Camera
Deodorant (
Ed: Please respect OTHER people's space by applying often 
)
personal ashtray
proper coffee (
Ed: Sorry, but I consider this essential, other may not 
)
ticket to event
Non-essentials:
Beer
Cordial
Fruit
New clothes for the daytime
Swimmers
Cooking food
Stove/Cooking Utensils
Poi/firesticks
Frisbee
Cricket ball/bat
Chairs
Esky - food/beer
portable table
plates - cutlery
towels
big lamp/torch for campsite
Tarps for under tent and general cleaner than sitting/laying on ground
Very Small Pen.
Decor for your campsite
Baloons (
Ed: Can I make a general note here about nangs and baloons etc. I know its popular at doofs, but its still considered very taboo in all but a select few groups, and at the larger festivals there are children, adults etc. Please nang in the privacy of your own campsite, out of view from the general public. Turning blue on the dance floor from sucking in a giant baloon isn't very attractive. Also, PLEASE clean up all those nangs afterwards. Have a dedicated bag for them.)
A bubble blower
A doof book with colured pencils and pens.... so people on acid can draw there crazy pictures and write down their spiritual revelations
Instruments- bongo drums, woodblocks etc. (
Ed: Please bongo politely. Nobody likes a guy bashing on drums out of time to the music, louder than the music)
A kalidescope
Tweezers
A camera and pen and paper
a safty pin to pin my keys to me (
Ed: Also those skate neck tag things with the removable clips are good)
bouncy shoes
Loads of Cigarrettes (complete with good ashtray. Don't want to be responsible for an early close due to bushfire, burnt cars and smouldering hippies!)
Good CD's
Spiderman costume & one for a friend
Super-Soaker (if not raining)
Hacky Sack
thongs/sandles
pre-mix cans for the morning
papers
marker so you know where your camp is
fine mist spray bottle
some kind of light signal to identify the car at night
preferably an inflatable armchair or otherwise a chair of some description
butter menthols / valerian forte / valium / 5htp / magnesium
scissors
Bottle Opener
someone to shag when it's all over.
backpack
sunnies