One thing that really helps, is smiling! I know it sounds stupid, but it actually works! At the Nationals when I lost my one stirrup while jumping when Damper half spooked at a jump, I was smiling and even half laughing a lot of the way around the course. In almost all of the photo's you can see me smiling broadly.
Another thing that helps is having a conversation with your horse, be it in your head or out loud. Sometimes, for example, if Damper randomly speeds up I'll think "Alright, you wanna get this over with, I know I know. Just focus for a few more seconds" or "I knew someone put rocket booster powder in your food". Something like that. It will take your mind off the nerves and all the other people, while still keeping your mind on the job at hand.
Below is one of the photo's of me smiling like an idiot. :roll: