I would have a look at using the utility that cspear linked to by D3D, it's ideal for setting up a mirror and placing objects so that they show up in the reflection, especially using the top view to align objects.

 

Here's an example scene. I have a room set up, and a chair that is to be in the mirror. This pic shows the view from my camera. You can't see the chair.

The green square is D3D's reflector, with a green line showing the camera view path, and another line from the mirror showing the reflection path. (The reflector is not green normally, I did it to show it easier)