Before you leave the house, cover yourself liberally in 8-hour suntan lotion, or else you'll get burned. Better yet, BUY a hat as soon as you get there. The odd sizes for people like me who have watermelon-size heads are gone quickly. Dress comfortably, and don't take an armload of stuff. Ladies, take the smallest purse you have.
Backpacks are good.
Less is more. If it's gonna be hot, wear shorts. Pants with pockets are a must. You're gonna be carrying around a number of small items, and having a place to keep them will prove handy. If the weather is threatening, bring a change of clothes and leave it in the car.