This one is kind of a no-brainer. It's summer, meaning the temperatures are scorching. With all the flailing around and sweating you'll be doing, hydration is a necessity. Festivals sell water, so make sure you have money available. If you want to save some pennies on overpriced H2O, bring a CamelBak or empty water bottle. Many festivals allow these and have refill stations. Commit to drinking eight ounces of water every hour.
Bring sunscreen that has a high SPF and lather up multiple times throughout the day. Not only will you protect your skin from a potential festival-ruining burn, but you'll also avoid awkward tank top tan lines.
An entire day (or more) on your feet, walking from stage to stage, can be exhausting. Take it easy in the afternoon and spread out on a blanket while watching bands. It's a good way to get recharged before the nighttime headliners go onstage.
For women, this could mean an over-the-body purse, for both sexes it could be a backpack. In my festival experience, I've used cargo pants to store my belongings. Fashionable? Nope. Functional? Yes. It all depends on your preference. If you don't mind carrying around a purse or a backpack, go for it. If you don't want to carry around anything bulky, I live and die by the hideous cargo pants. You can also go for the fanny pack look. At least that falls into festival fashion.
Do I really need to explain this one?
It's going to be hot, dirty and you're going to be walking more than you've walked in the past three months. Don't let blisters get in the way of these already-uncomfortable hurdles. Wear shoes that you know you can spend an entire day/weekend walking around in. Don't wear new shoes, but if you must, spend the week before the festival breaking them in.
CHAPSTICK WITH SPF
Don't forget your lips! These can burn just as easily as your skin. Protect those smackers.
You will eventually have to use the Port-A-Potty.
The last thing you want is sun in your eyes while you try to watch your favorite band. Make sure you get the best view and throw on some shades.
In case you didn't listen to the "sensible shoes" advice.
A lot of festival vendors won't take plastic, so bring enough cash to get some food and beverages.