Make sure the source of light is behind the subject.
Make sure there are no obstacles in the background or foreground that will obscure the shape of the silhouette.
Make sure your camera settings are correct for the time of day, available light, etc.
If it is dark and the shutter speed goes below 1/50 of a second, use a tripod to steady the shot.
We were on our way home from a senior shoot and saw a beautiful sunset over Grapevine Lake. We parked the car, and ran up the hill to get a few shots. This one is SOOC... Straight out of the camera.
For more photos, click here to get to my website or Facebook page!