The trick here is to place the camera on a different axis than the subject and the mirror. In most cases the person looking in the mirror will not be able to see themselves, at least not in the same manner that they appear on screen.
Position the camera and the mirror so that the camera is just out of the mirror field of view, as seen through the viewfinder. Then position your subject so that they appear to be looking at themselves in the mirror, as seen through the viewfinder. Again, they will probably not be able to see more than a small portion of themselves, depending on the size of the mirror. They may not be able to see them selves at all. They will, however, be able to see the camera, and it is very important that you direct them not to look at the reflection of the camera, as that will record as them looking directly into the lens.
A good trick to shoot a close-up cover of the subject looking directly at the camera and applying makeup or primping as though the camera were the mirror. When cutting this together with the shot above, and perhaps another side angle, this will complete the illusion that they could actually see themselves in the mirror while your were shooting.
