There's really two scenarios to plan your outfit around:
1) you are going to the festival and are NOT going out anywhere afterward
2) you are going to the festival and ARE going out somewhere afterward
Here's a practical yet fashionable outfit for the first situation.
Shirt: No name
Shorts: Forever 21
Bathing suit: Ritchie Swimwear
Crossbody: Dooney & Bourke
Necklace: Forever 21
What's important is staying dry - wear undergarments that will wick off sweat (a bathing suit). Wear a loose fitting shirt and shorts to stay as cool as possible. Make sure you have a crossbody bag so you can feel free to pump those fists and dance without worrying about your stuff! Wear sneakers so that your feet are comfortable all day and don't get too muddy.
Here's an outfit that is slightly less comfortable yet serves a dual purpose for the second situation:
Shirt/Mini Dress: American Apparel
Belt/Headband: American Apparel
Loose Powder: Elf
When you know you are going out afterward, an easy way to transform your day look to a nighttime look is with this versatile dress/shirt from American Apparel. Just take off your shorts, pull down the shirt so it stretches to its fullest, and you have yourself a mini dress! During the day you can wear the belt around your head as a headband to keep your hair out of your face, and at night you can wear it as a belt to add some definition to your dress. While I wouldn't say makeup is necessary when you KNOW you are NOT going out at night, I know for sure I wish I had brought powder with me 2 years ago because I looked like a sweaty mess by the time the festival was over. If I had to suggest 1 makeup product to squeeze in your bag, it would be loose powder. You can also get away with sandals, but only a fool would wear high heels to an event like this. The clubs tend to be more lax about the dress code during music festivals, so you could also get away with wearing sandals to a club instead of heels, especially if you are wearing a hot mini-dress above those sandals :)
Things to remember:
1) No "professional" cameras allowed (usually) - you can bring your DSLR but do it at your own risk.
2) You will get dirty. If you want to sit down the only place to sit is the grass.
3) Hydrate yourself before you go. Water (and other beverages) are extremely expensive but if you feel you need water, SPEND THE MONEY. No one wants you to get sick!
4) Remember sunscreen.
5) Wear clothes you don't necessarily care about because of said dirt and sweat.
6) Only bring "essentials" - for me this is my phone, a small point and shoot, a wallet.