1. Stay hydrated and sated. You don't want to be in a situation where you have to make split-second decisions which may involve running, jumping, shoving or fighting on an empty stomach while dehydrated. Plus, dehydration and heatstroke are the most common reasons for medical intervention at outdoor concerts, so you need to be careful to stay hydrated, stay cool, and keep at least a little bit of food in yourself.
This will mean bringing cash, as many food booths won't take debit cards and the concert ATMs will most likely try to gouge the living shit out of you with processing fees and other nonsense. Also: The hotter it is, the less fashionable you are allowed to be. Your leather jacket might look awesome, but leave it in your car because it will feel like a heat-soaking murder blanket all day.
You may have spent hours getting your hairdo just right, but in 110-degree weather, dumping a bottle of water on your head is a lot better than passing out in the mud and getting trampled to death. There is no common sense in fashion, but there's plenty in survival. Unless you're one of the unlucky people who actually buys into that "live fast, die young, leave a beautiful corpse" stuff, then go nuts, you gorgeous idiot.
2. Don't get packed in too tight. We know it's tempting to get right up in front of the stage just in case indie heartthrob Jeff Tweedy tosses out a pair of his boxer briefs or something, but not all concerts are safe for this kind of thing. Look around you. Are there an adequate number of barriers and security personnel?
Sure, usually security personnel are just there to make sure you don't bring in any food from outside the venue, but in the rare case when you get knocked down on the ground or get in a fight, they'll be the ones intervening to keep people from getting too hurt, so unless you're one of those jackasses who plans on rushing the stage, try to have one or two nearby. Keep a barrier close by, as well, so that if the crush of people gets too intense, you can hop over it and scurry to safety.
Suffocation/trampling is one of the uglier ways to die, so try to keep a level of awareness on how crammed in you're getting, and always try to have a quick way out. Failing that, stand in the midst of very good-looking people, so that if you are squeezed to death, your last few moments on earth will at least be kind of sexy.
3. Bring or meet up with friends. It's simple: not only are festivals more fun with friends, but they're safer, too. It's nice to have folks around who will come to your aid if there are problems. Just don't bring your friends who are always getting drunk and starting trouble. Festivals are really, really not the place for that crap. Drop 'em off at a frat party and enjoy the show.