Identify your body language patterns. Make a conscious effort to think about what your body is doing in different interactions with different people. A mirror can be useful to examine facial expressions and posture, but mainly you just want to pay attention to what your body does when you're angry, nervous, or happy.
Determine whether your body language is in sync with your message. Your body language is effective if it communicates the message you want it to communicate. Does your posture communicate confidence, or does it make you seem unsure of yourself even though your words express confidence?
If your non-verbal signals match your words, you'll not only communicate more clearly, you'll also be perceived as being more charismatic.