Practice the piece you are preparing in front of a mirror. This allows you to see what your audience sees. You can also tape yourself and take notes about your own performance.
Work on good posture. This is basic to any type of communication. Good posture allows you to breathe more deeply and relax.
Observe your facial expressions in the mirror. Does your face reflect what you are trying to communicate? Make your face reflect the mood you want to convey to your audience.
Consider your body language. A poised, erect body appears confident and in control. Body language will betray what you say if you aren't fully engaged in your presentation.
Use gestures at appropriate times and with purpose. While you don't want to look like an uncontrolled bird, a few purposeful gestures will help both you and your audience relax.
Use language appropriate to you audience. If you are speaking to a room of seventh graders, you'll use different words than for a group of senior citizens. Consider your language carefully and speak at their level.
Make them laugh! There's nothing that will endear you to an audience more than helping them laugh a little. Even if your topic is serious, a little levity connects people in an amazing way.