Another aspect of your appearance is your body language and movement. Many people get nervous when the camera is turned in their direction. Knowing what's in store can help diffuse some of the nervousness, and there are ways to appear calm on TV even when you are nervous. Avoid large and quick movements. Slow, deliberate gesture are best, especially when demonstrating a technique or practice.
Slow movements are much easier for the viewer and camera operator to follow.
More times than not, you'll be interviewed by someone. If you concentrate on looking directly at that person and can block out the peripheral activity, you'll appear much more calm and collected. Perhaps the best strategy for appearing on TV is to limit the amount of time you're actually on camera. The best way to do this is to supply visuals interviewers or producers can use. Some studios have the ability to incorporate slides into interviews.