Always buy tickets from official websites. Skeezy looking ones can prove to be major rip-offs. Also be wary of buying from scalpers. Not only are they overpriced, they may misrepresent some crucial details.
At multi-stage festivals, there are always two or more bands playing at once, as well as other side events. If you want to get the most out of the experience, you need to keep a tight schedule to optimize your fest time.
If the festival allows it, always bring your own food in. Festival grounds can charge some truly ridiculous prices for small amounts of food. No one should pay $13 for a small, tasteless pizza. No one except Nickleback.
Stay hydrated. Because bottled water is expensive, you always want to bring a refillable bottle such as a Nalgene and keep yourself hydrated. Passing out in the hot sun means you miss your favorite band.
Look, there are all sorts of people at large festivals (small ones, too). Be safe. This means not leaving valuables in sight, not following strangers to their campsites, etc. And I wouldn’t eat the mystery brownies, either.