Make sure you remember where your tent is. Orientate yourself with a land mark - not one that moves like another tent!
Take some decent food with you. It's a seller's market so festival food is almost always overpriced, and lousy value.
Have a plan for who you want to see, and be in place in plenty of time.
Take some earplugs and a sleep mask unless you plan being unconscious through alcohol by bedtime!
Take a good paperback - sometimes a little escape from the constant noise overkill is welcome.