Have a small but protein-full breakfast (something that will sit in your stomach without making it upset!). Also, pack simple snacks like pretzels and lots of water!
Make a list of the things you'll need for show day. Being (and feeling) prepared is half the battle!
Give yourself too much time to brush down, tack up, and warm-up. Chances are it'll turn out to be just enough. ;)
Smile and just think of how lucky you are! Whenever you begin to get stressed out, exhale and smile. Plus, judges love to see a smiley rider!
Take pride in every little thing-- small successes are way better than ribbons. Best of luck!