Well, you're already following two of my suggestions, which would be to get lots of practice at it (do a lot of comps and showcases), and to be well-prepared. Try this: as you're standing in the staging area getting ready to go on, close your eyes and take a deep breath. Visualize yourself getting through the whole heat without getting uptight, staying calm and relaxed throughout. Every time you start to feel tense, take a deep breath. Hunch your shoulders up to your ears, then relax and let them drop. When you get out on the floor and frame up, take a deep breath while in dance position...this will make your partner take a deep breath, and you'll both relax. Remind each other during the dance to relax and breathe.