There are a couple mixed breed associations, and some of them hold shows that include mixed breed conformation classes...based on soundness, temperment, manners, and overall shape of your dog. I believe you can search for different chapters in your state...here's some links for those who are interested.

Mixed Breed Dog Clubs Of America
American Mixed Breed Obedience Registration
North American Mixed Breed Registration

I don't have experience in any of these, I only know of them, so I can't tell you much more.

I don't show any of my dogs-two of them are mixed, and my GSD was born from a stray dog. I would love to do it though..I want to show a Dobie.