Make sure you have your basic necessities, brushes and tack, pack an extra halter and leadrope just in case. Have your boots, show clothes, helmet, and whatever show necessities you need. Bring leather cleaner to go over your boots with. Depending on the weather, bring a cooler to keep your horses muscles warm. Bring a bucket so you can keep your horse hydrated.
Buy some baby wipes or a sponge you can wet. They are great to finish off their coat if it gets dusty. :)