The key to a good silhouette is simple. GET DOWN LOW. You want your subject whatever it maybe to be like a wall separating you from the light source. The lower you can get from your subject the more defined it's going to be. Much like Sunset photos silhouettes are about exposing the sun which mean a much higher shutter speed then you may think. If you don't know what shutter speed is try checking out my tutorial on Shutter Speed here and for just kicks it wouldn't hurt check out my tutorial on Aperture either. When shooting a silhouettes you're almost always shooting into the sun (or light source) and that sun is a lot brighter than you may think to your camera's sensor, so raising the Shutter Speed, and Aperture, while lowering the ISO is going to set your camera in the perfect condition to line out your silhouette. Try it. They're easy to get addicted too.