When shooting silhouettes, remember that recognizable subjects are vital to your compositions. If the shape of your subject is too abstract, the image may not be as engaging (or recognizable) to your audience. For example, if you want to create silhouettes of people, encourage your subjects to wear dark, form-fitting clothes. Light-colored clothes make it harder for you to capture very dark shadows, and loose clothes are a distraction because they make it more difficult for viewers to recognize human forms in your composition.
Be creative with your pictures. You don't always have to center the silhouette; you can use dark, shadowy edges to frame your primary subject. Or you can incorporate a silhouette on just one edge of your picture -- doing so will shift the weight and balance of your compositions, giving each picture a very different visual aesthetic. Don't worry about rules; just keep trying new angles and perspectives.
The further you push your experiences, skills and imagination with regard to silhouette images, the more fun you'll have. So, no matter which subjects you decide to use for your silhouettes, experiment and don't be afraid to try wild ideas (even if they might not result in usable photos). The more you pit your picture-taking knowledge against tricky lighting situations, the more competent you'll feel with your camera and photography in general.