When listening to an opponent speak pay attention to your body language. Studies have found that when a nonspeaking debater expressed continuous disagreement or disbelief in what their opponent was saying, by frowning, head shaking, mouthing rebuttals... the audience tended to view them as deceptive and the speaker as truthful. Some times speakers will use negative body language in an attempt to sway the audience to their point of view.
Studies like the one above indicate that if you are unaware of your body language you may be inadvertently influencing the perception of your audience - in a less than desirable direction. When listening to others pay close attention to what you may be inadvertently communicating - ensuring that your body language is delivering messages of your choosing!