I just take deep breaths and focos on the horse remember they can feel u tenseing up so just block out everything and do your best
Bananas help calm your nerves (Don't know why but they do)
I like to tell myself that it's just another practice ride.
Here's what you can do with no order of impotence. Just pick what you think works for you:
Good night sleep
Night before Make love
Night before Massage
Yoga in the morning before the show
Do something isn't related to horses or at the barn the day before the show
Which one you think you probably going to do ?