I know this isn't exactly what you meant but just remember to have fun and realize that you only need to compete against yourself. That way there's less pressure on winning and more on enjoying the experience. Good luck!
Focus on deep breaths. Have a good healthy breakfast, nothing to greasy if you think you'll have an upset stomach. Leave in plenty of time and get there early so you can have a look around. Keep an eye on the PA or stick around your ring so you can see if they're on schedule. And get you and your horse out and ready a little early so you can both calm yourselves.
After that just have fun, horse shows are supposed to be enjoyable!