A lot of people around here put a canopy next to their trailer and let their horse(s) stand under it. Also, move your horse with the position of the sun. In the morning and evening there should be sufficient shade from the trailer alone, but as soon as afternoon hits is when she'll start to suffer.
Also, keep lots and lots of water available. Keep offering it. I bring electrolytes to shows, and really keep my horses hydrated. After learning the hard way about not keeping an eye on my horse's heat level enough, I always make sure he is comfortable.
So for showmanship, I think you'll be fine. Get there extra early, lunge/warm up in the morning cool, and then wait for showmanship.
But most of all, have fun (: I miss showing my guy. Oh, and pictures are a must (;