I love festivals! I think I've been to around 10 now (yea i really love going). However I go in the US where it is super hot outside. Usually I just rock some cutoff shorts and a breezy tank top or even a bathing suit top. At least here, it doesn't look that out of place. I also take a camelbak (in case you don't know, its a backpack with a water pack in it) and that way you have access to water, plus you can put a small blanket in there along with sunscreen, camera, ect.
If you take your phone or non disposable camera, I always put mine in a plastic bag in case it rains. A good cheap pair of funky sunglasses is a must too. Sandles can start to hurt my feet if I'm walking around in them all day, so i like to wear a worn pair of sneakers like converse or just some slips of some kind.
I was actually thinking of trying this out on my hair this year what do you guys think:
http://vimeo.com/39663022
sounds like you will have a blast, just go with the flow and you will be fine.