You'll need to keep cool and comfortable. Wear shorts and dress as lightly as possible. Be sure to bring a bottle of water with a replaceable cap in order to cool yourself down while standing in the hot sun watching 30 minute set after 30 minute set. Wear sunblock, and try to stand in the shade.
In order to survive the Vans Warped Tour, you'll need to be aware of where you are at all times relative to the mosh pits. While intended by many as a fun way to express their enjoyment of the music, moshing is a very violent activity for others. This threat is intensified by the number of people around and the intense heat. Check the schedule posted at the entrance that lists where and when each band is playing. Make yourself aware of the type of band playing on the stages near you. Stay away from mosh pits unless you feel that you can handle being constantly pushed and thrown around.
Be aware of the people around you at all times. If somebody bumps into you in a strange way, check your stuff and make sure it isn't missing. Be friendly, but don't make yourself vulnerable and/or an easy target.