How can I avoid nervousness during military ceremonies where high-ranking Service Members are present?

Last time I checked, I still put my pants on one leg at a time. Enjoy the tradition and show.

Most of them were in your shoes in a similar position at some point. Most of them remember that time too. Concentrate on your duties and screen those nervous thoughts out. Treat ceremonies like the best practice you've ever had, and also remember once it's done, it's done. Even the Blue Angels make mistakes. They review afterwards, state they'll do better next time, and keep pressing. Finally, remember you've passed the test and been selected for the duty. You belong there and will do fine.

Flag officers are to be respected, for sure. But unless one is being an asshole, there isn't any reason to be so nervous that it affects you physically. They're just people too. S(he)'s gonna go home that night and eat dinner and argue with the spouse and get exasperated at unruly teenagers, just like you do, or will. If you perform your duties as you're trained and expected to, and salute like you're supposed to, then you've done everything you're supposed to, and needn't fear.