There are a couple of things to look out for in black and white. One is the importance of "value" in your subject's clothing. For instance, let's say you had a well tanned actor with brown hair, wearing a red shirt, blue pants and standing in front of a green wall. Obviously, on color film you can see him fine. However, in black and white, all those colors -even his skin and hair- will be grey. If they are all accidentaly the same "value", then they end up being the same shade and density of grey and the guy will disappear in front of the wall!!! Not literally, of course, but you get the idea. I would suggest taking photos of your actors in their clothing and using photo shop to desaturate the color to see how they will look before you start shooting.
Now, related to this is the use of a rim light to help seperate the actor from the background. This can sometimes be useful because, while you can always change the actor's clothing, you can't always change the background beyond lighting adjustments. Using a rim light helps in black and white to add definition to the edges of an actor that, otherwise, would be lost due to a lack of color to tell the eye where the edge of the person was in the scene. It also looks damned classy.
Roger