1. Allowing your eyes to wander around the studio
If your eyes flicker around during a TV interview, you look uncomfortable, and probably guilty. Keep your eye-line focused on the interviewer, then you come over as being in command of your subject. Avoid frowning - it makes you look miserable. With most topics, a smile won't come amiss!

2. Saying 'No comment'
The viewer or listener is likely to jump to the conclusion that things are so bad that you haven't got any line of defence at all.

3. Assuming your audience knows much about your subject
You would be surprised how ignorant the general public can be! Phrases like 'as you probably know, our XYZ386 model last year was very successful' are a turnoff for all but the cognoscenti. People often make the mistake of addressing their remarks to the interviewer (who may be well-informed) rather than the audience.