Get a Sharp Silhouette
Of course, you want the subject in sharp focus, even if it's in silhouette. If your subject is far enough away, that isn't a problem: When you lock the exposure on the bright background, the camera will set the focus to infinity, and you'll get good results. If the subject is too close, however, it will be in a different focal range than the background, and the silhouette will turn out fuzzy.
To avoid the problem, check your camera to see if it has a way to lock the exposure and focus separately. If it does, use that mode to prevent the focus from locking in on the background when you set the exposure. If it doesn't, switch your camera to manual focus, and focus on the subject by hand; be sure you've locked the exposure, and then recompose your picture and shoot.
If all goes well, you'll get an inky-black subject. If your subject isn't quite silhouetted, try again, underexposing the image even more by using the exposure compensation control on your camera.