Get there EARLY so that you can groom groom groom. Even if it is cold, you can shampoo a dirty tail in a bucket of warm water and towels.
The horse needs to be as spotless as you can possibly get him. Bring your own clean saddle pad, if you have one. Be prepared to clean the tack of the horse you are riding. Your clothes need to be clean and conservative, your hair contained. I know, it sounds nit-picky.
When I teach a clinic, I can't help but form "first impressions" of the rider in front of me (I never do groups). I try to be lenient when it is subzero, but..........it is a matter of respect.