Shari, unfortunately many people share your sentiment - I'm included. Even when you can't find the right words to say, your non-verbal communication speaks volumes. I think it's a good practice to try to wear a smile; however, the danger in that is smiling when someone is being very serious. Then the smile becomes offensive. So I have a professor to give me a few pointers that you might like to try. Give eye contact (don't stare), nod in the speakers direction whether you agree or disagree, stay engaged and if you smile don't show your teeth. I thought it was a little ridiculous until I followed the instructions. The first comment I received was, "We can't tell how you feel about this subject because we can't read your expression", BINGO!