Several things I use to calm down.

1. Breathe deep. I don't mean that pansy stuff you typically do, breathe deep. Close your eyes and suck in air until your chest expands and you can't take in anymore, then exhale slowly. I always do this when I need to calm down, and it works pretty well.

2. When you become lucid, pause, and concentrate upon the first thing you see. Don't look at anything else, don't think about anything else. Just lock your eyes on whatever it is and study it. Touch it, smell it, etc. The idea is to ground yourself in the moment, so that you don't get so anxious you lose your lucidity.

3. Before sleeping, tell yourself that you will remain calm when lucid. Autosuggestion can't hurt.