The subject should be something with a distinct shape, easily recognizable. Horses, people, even a palm tree on the beach, can make interesting subjects when silhouetted against a sunset. Normally I cringe when I see some travel photos where the poor subject, usually the photographer’s better-half shows up in the picture underexposed, yet the background is crystal-clear. It may be a bright and sunny day on the beach or posed at some easily recognizable attraction but, the poor human subject has their face shrouded in dark shadows. I tactfully remind the photographer it would have looked a lot better had they used their flash.
So, what I mean by breaking the rules is deliberately setting your camera exposure for the bright light. That’s exactly the opposite of what I’d normally do. You will not adjust your settings for the best exposure on your subject, you will be underexposing and creating even more shadows on your subject.
Just how much to underexpose is entirely up to you so, it may take some experimentation. This is one time you may want to let the camera decide, just by putting it in AUTO MODE. Then, review the shot on your LCD screen and decide if you’d like to darken the silhouettes, even more. A faster shutter speed or stopping down on the aperture will make the shadows more dramatic, totally blacken the subject, if you like.
A few points to consider are the position of the subject in relation to the bright and dark spots in the background and the profile of the subject. If the subject is a person leaning on a tree, rock or building and it is in the shadows, as well, underexposing will blend them together and you’ll loose the shape of the subject. You want the subject, as a silhouette, to stand out.
The profile of the subject will be more dramatic if your composition is from their side, not head on; this will make their nose, eyes and lips stand out more than if you were to attempt a frontal portrait shot.
At festivals, after a long day of shooting in the bright sun, I take silhouette shots. When there are bright lights in the background and I don’t want to be bothered chasing subjects down to get their permission. Just hide in the dark and shoot. After all, that’s what silhouettes are for, breaking all the rules.