Silhouette images can be pretty cool. This is because the high level of contrast between lights and darks, often combined with saturated colors, makes an immediate impact on the viewer. However, this visual impact makes it easy to let a mediocre image slip into your portfolio.
For this reason, I hold my silhouette images to a higher standard when it comes to the shape of the subject. The subject must have an interesting graphic shape, such as in the picture below:

Two women in silhouette on beach outside XiaPu, China
This image works well because the dancer (a.k.a. my wife) knows how to pose well.

First Light : Prints Available
Dancer exercising at sunrise on Lake Michigan
Depending on your shutter speed, you will most likely need to use a tripod and also ensure your subject is completely still. None of the edges of the silhouette should be blurred, unless you are blurring for effect.

The Fountain : Prints Available
Elegant woman watching a fountain during a brilliant sunset
Also, in many cases, you don’t want the silhouette to go completely dark. Including a little detail in the shadows often works better than having a completely black shape. In the image below, I brought out some of the details of the lighthouse in post-processing.

All is Well : Prints Available
Silhouette of two men fishing off a lighthouse pier during sunset