Ok, I'm not a percussionist, but definitely bring your practice stuff (pad, sticks, and whatnot). And definitely bring a good attitude. While your at it, bring some good chops. The staff is nice, but they won't cut you any slack. The percussion staff is definitely known for that.
In conclusion, come prepared and ready to give it your all. I guess you usually get a spot with that...
P.S, like everyone else said, don't be cocky, but don't put yourself down. Be cool and confident.