I get very nervous before I show so I find keeping myself busy before hand is always good! Once I'm actually competing I try and think about good rounds my horses and I have had and channel that positive energy!
sing the song that's playing
Sing your favorite song and focus on the color of the jumps. And prevent your the horse. This how would he run towards it, jump it, land it, be him/her. I always do that. And focus on the feeling of your horse
Hope this helps