Digital Photography School offers eight things to keep in mind when you attempt to capture a silhouette
Aside from the technical tips on focusing, lighting, framing, and metering, they make useful suggestions about subject. After all, just because you can make a silhouette of a subject doesn't always mean you should. Here are two of our favorites:
1. Choose a Strong Subject** Almost any object can be made into a silhouette, however some are better than others. Choose something with a strong and recognizable shape that will be interesting enough in its two dimensional form to hold the interest of those viewing your image. Silhouettes can't draw on the colors, textures and tones of subjects to make them appealing - so the shape needs to be distinct.