You can & should wear whatever you want. Hit up thrift stores, relatives' old unwanted vintage, theatrical costume shop sales, army surplus, love boutiques, dollar stores, etc. Fill in the holes with stuff you find at actual costume shops.
Look at the 2011 Theme Camps on the main site and see what kinds of theme nights and events they're having, as you might enjoy having Just The Thing for some party . . .
Here are the arguable essentials:
Comfortable, broken-in shoes or boots
More socks and underwear than you technically need
Goggles (best: one pair dark for day, one pair clear for night. goggles designed for soldiers tend to be good).
Warm coat for night
Lightweight stuff for day (jeans are uncomfortable for some; Black Rock City is the most skirted city in the world)
A wide-brimmed Boonie hat or cowboy hat
Bandannas, pashminas, shemaghs or wide scarves to wrap the hair and the lower part of the face
Lights to wear on your body, front and back, so no one runs you over with an artcar (it's happened)
Make sure that trim is soundly attached, as you mustn't drop buttons, beads, jewels, or sequins (or similar), & don't bring feathers (except maribou, according to the main site). The rest is up to you. There are some obvious Burner clichés--though they are popular for a reason. Tutu Tuesday is a fun one.
There are a million threads about gifts. I still don't use the Search function on the site, so I typed only gifts eplaya
into google & got a bunch of hits. Give it a shot. One of the principles of the event is radical self-reliance, so I'm going against my natural tendency to tell you everything I know.