If possible, get in the ring before the show starts. A lot of places will let you warm up in the arena you show in early in the morning. Mind you, some places won't, but if you can, then do. That way you know the layout of the ring, and you know what your horse will think is going to eat him.
Make sure and stay hydrated. I know some people get so nervous they don't drink enough water(not soda!!), and they get very fatigued. Which is no good when you need to concentrate.
As for the nerves, its going to happen. Lol I've been showing for seven years and I still get nervous!!! Take a deep breath...keep breathing...And if you have a routine that you do before shows, do that. I always listen to the Eagles driving to shows.