(Please excuse image quality and smirks. This isn't a photo shoot or a fashion show we're talkin' about here.)
Keep in mind that this is 90% for any festival that isn't glamorous. If you're there for the street style photographers, I guess just wear what everyone else is wearing at x brand's Coachella pool party.
DO NOT WEAR
Flip flops / This basically applies to any shoes you actually care about or that don't cover your feet. For real, have you ever been in a port-o-potty? Also, bring rain boots. If you don't bring them, the chances of monsoon go up 100%. It is known.
Rompers or overalls / I repeat, have you ever been in a port-o-potty? You do not want to get basically naked in there - unless that's your cup of tea. In which case, be courteous to the line waiting behind you and make things quick.
Anything rude or annoying / Including but not limited to: Indian headdresses, duct tape nipples, high heels of any variety, shirts of bands you don't actually listen to, fur that is molting on those around you.
Sunscreen / By far the most important takeaway here. You don't want to end up like this guy.
Things with pockets / Denim rules supreme. You're also going to accumulate a lot of stuff and you'll need to stash it. I strongly suggest a backpack or fanny pack if you don't want messenger-bag-sore-shoulder syndrome.
A good attitude / You're tired and dirty, everyone else is tired and dirty. Be nice. You'd be surprised at how good you'll feel sharing a simple luxury with a stranger. (Pack your bag very thoroughly. When in doubt, you definitely need extra tissues/sunscreen/water/etc).
Whatever you want / No matter what, there will be someone dressed crazier than you. (Shoutout Pink Jesus, Pitchfork 2010). This is the time to let your freak flag fly. Be you. No one else cares.