The real issue here is that your camera has no idea what you are taking a picture of muchless what you want it to end up looking like. To determine how light or dark your photo will be, it's programed with a very simple goal in mind -make every photo average out to a pre-defined "average" tone (about the same shade as the palm of your hand). This works well most of the time, but not so much when you have a lot of really light or dark tones in your photo.
Here's a different way to put it: Your camera assumes that if the photo you are about to take a picture of was blurred 100%, it'd come out to be the same tone as the palm of your hand (tone = how light or dark the resulting color would be).
So, when you take a photo that's very light (like a snowman in a snow covered field) the camera has no idea why it's so bright and will darken it up so it comes out looking gray instead of white. Or when you take a picture of something dark (like silhouettes) they tend to come out lighter than you wanted (they come out gray instead of black) because the camera has to tone down all that black to make that "average tone". So, either way -with really light or really dark scenes, everything moves towards this medium-gray, but how do you fix it? It depends on what kind of camera you have!
FOR POINT AND SHOOTS OR SLR'S WITH EXPOSURE COMPENSATION:
Check your camera's manual to see if it offers exposure compensation in the mode you're using.
Exposure compensation lets you tell the camera to make the photo lighter or darker without jumping through hoops to "trick it" into doing so. Each camera works differently, but you should be able to either move an indicator on a graph that looks like this:
[-2...-1...0..+1..+2] (Nikons have the positive numbers to the left, but it doesn't really matter.)
or change the value of a number ie: "EV +1" or "EV -2".
When you move the numbers towards the negative side you're telling the camera to make the photo darker. Positive numbers mean the photo will come out lighter.
So, for silhouettes, you'll want to try maybe "-1" and see how that works. If it's still not dark enough try -1.5 or -2 -just keep playing with it 'til you're happy.
FOR POINT AND SHOOTS WITHOUT EXPOSURE COMPENSATION:
To get around your camera's "average tone" assumption, you'll have to trick your camera. If you want it darker, point it at something lighter -like the sky! Point your camera at the sky just above or next to whatever you want to be in silhouette and then push the shutter half-way to lock in the exposure. Then re-frame the shot to include whatever you want to appear in silhouette and push the button the rest of the way down to take the picture. (Pushing the shutter half-way usually locks in your focus as well as your exposure, so if your subject is close it may come out blurry -just experiment with it to see what distances work for your camera).