Posture

Boxing has been a great help in getting at paralanguage. I find it rather easy to concentrate on the body language of my opponent when knowing I could take that right hook on my chin.

Besides the overall contribution one of the primary issues where I gained better understanding has been posture. Whether the opponent is leaning back or forward or whether I'm leaning in some direction. Whether I'm facing him directly or with a slight angle.

Posture can play a very important role in reading body language as it is one of the easiest signs to notice.
When talking with someone you can easily notice if they are leaning towards you which my be a sign of interest. Leaning backwards can be interpreted as "not really interested" or it can mean that the other party feels comfortable (it can even carry a hint of "territorialism" in a sense they feel at home).

You should also keep track of your own posture as any message you send with your body is received by the other party. These message won't always be interpreted but they will always be received and create a "gut feeling". So when seeming interest is to your advantage (a prospect, a flirt, or a friend) try to slightly lean towards them. This will also help create trust as you will be slightly getting closer (think proxemics) without entering the intimate area with your whole body, a rather nice compromise I'd say.