Practice neutral body language and facial expressions, so that your sitting rigidly and/or frowning during a meeting is not misinterpreted as anger, disagreement, or boredom, when you may be only concentrating.

- Brown, Haygood, and McLean, The Little Black Book of Success

Have you ever had someone ask you what was wrong, and you were surprised because you were feeling completely fine?  Sometimes, our default "focused" appearance can accidentally come across as upset or angry.  The best way to catch this is to ask your family, friends, or colleagues to alert you when it seems to be happening.  If they can take a picture without your knowledge, this can be an excellent way to get to see it for yourself.

Also, make a concerted effort to take a moment at various points during meetings to tell your body to relax your shoulders, your hands, and your facial muscles.  You may be tensing them without even knowing it.  The more you practice doing this, the less you'll find yourself having to forcefully relax to create a more open and neutral impression.