Discover new music. Festivals are all about seeing as many artists as possible, in a short amount of time, and this exposes you to artists you might not know. Check out an artist's set you might not ordinarily go to - you may be pleasantly surprised!
Pick a meeting spot. It's so easy to get lost at these huge festivals - it probably WILL happen, especially if you're in a large group of friends. Picking a meeting spot will make it easier to find each other again.
Use a real camera (if you have one) instead of your phone! You may want to whip out your phone and start snapping, but this also eats up your battery! If you have one bring a point-and-shoot - you can upload pics to Instagram later. #latergram
Use your phone sparingly. If you keep texting/instagraming/tweeting/facebooking the whole day - #1 you'll miss the whole festival, and #2 your phone battery will die, and die fast. Then you are cut off from the world. :(
Wear sneakers or closed toe shoes. Sandals or flippy floppies may look cute, but your feet won't look cute by the end of the day. To avoid "festival feet" choose a pair of old sneaks that you don't mind getting ruined.
Get that hot girl's number! Or ladies, go talk to that cute guy! You never know what relationships may form from these festivals. At the very least you'll end up with "the festival relationship" i.e. you spend a beautiful day together, and go home with happy memories.
Bring a towel or old sheet to sit down on. We know, you want to dance your booty off. But if you're there from beginning to end, you will exhaust yourself. Take a breather and rest for a few. Plus sometimes it's nice to sit and vibe to the music depending on what you're listening to.
Bring an extra change of clothes just in case. In a perfect world, it would be sunny and warm during festivals all the time. But what if it rains? An extra change of clothes could really save you/make you feel like a human again if you're sopping wet from a rain shower - or you just feel gross having sweated during the day.
Bring extra money and hide it somewhere you'll remember. Extra money for a cab ride home, or a water when you're dehydrated, or food to eat when you're starving can be a life-saver - literally. Not to mention, cash is accepted everywhere, unlike plastic. Bring an extra stash of cash people!