When I first got my D200 I found that I would periodically bump the focus selector switch on the camera body. When I did a bit of research on the web I found that many others had the same problem. As far as I know the only solution that anybody ever came up with is to be more aware and more careful when picking up and handling the camera.
If the switch you are having trouble with is the one on the lens, you may be able to cover it with a piece of tape. ...but I have never heard a definitive solution for hitting the switch on the body.