Is your avatar picture at the Buckeye Horse park in OH, by any chance? Looks familiar; that's where our regional rally is.
Above all, make sure to pack lots of hay strings (the PC approved kind, not the plastic ones) for breakaway hooks and label EVERYTHING with your last name, team name, and number. Make sure you have the appropriate number of free holes above your bridle and girth buckles. No rust on hoof picks, have paper bags for grain, and feed labeled in pounds and not scoops or quarts. Pack lots of bedding if it's not provided, and triple-check your kits with your teammates. They love to get you on expiration dates on sunscreen and triple antibiotics creams.
Also, be nice to your stable manager! It's a hell of a job to take care of 3-4 horses during the rally, get kids where they need to be, get ponies in the show ring, to equipment checks, safety checks, turnbacks, help them memorize their tests and courses, etc. It's a really tough job, so try to help out with your own horse as much as you can.