You really want to be aware of yourself and your position. Get those heels down and that back straight first and foremost. Be sure to keep short reins and look where you are going. If you can, try to get in front of the judge as much as possible and do not let another horse get between you and the judge when you are riding right past him/her. You don't want their line of sight to you blocked by someone else. Of course, be extra attentive to your diagonal and the horse's lead when cantering. Eyes up, hands forward and quiet. Most of all....breath!
Also, make sure your appearance is very neat and clean. I polish the living heck out of my boots and I ask someone to wipe them clean right before I got into the ring, especially the bottoms. Every little detail counts. Have fun and try not to look angry, I do that sometimes because I am really concentrating. Haha.
I always had trouble with my heels...I found it really helpful to spend a few minutes every day with the balls of my feet on a stair and just stretched out my calves that way...did wonders for dropping my heel while riding, I started placing much better in the flat classes after that!