At its root, nervousness stems from feeling like you have no control over the outcome. Once you get to a show site, you don’t control the judge, you don’t control the venue, you don’t control whether your dog is actually going to do the things you trained him to do, you don’t control the other competitors, and there’s ALWAYS something that you forgot or didn’t know enough to train.
If there is anything you can do ahead of time to be more prepared and make the outcome more predictable, do it. If there’s nothing within your control that you can do, accept it.
If you start feeling nervous, ask yourself what is making you nervous. You may or may not be able to answer the question, but if it’s something you can’t control, mentally file that under the “not my problem right now” folder and let it go.