It's fairly simple, really. In the materials room, select the material zone for the mirror. Then, turn off the 'reflection_kd_mult' and 'reflection_lite_mult' options at the bottom of the root node (the checkboxes). Set diffuse colour to black, and disconnect any nodes that are coming out of it. Add a reflection node to the 'reflection colour' input, and make sure your reflection value is set to 1. Make sure you either have a complete scene to be reflected or, if you have gaps behind the camera, then plug in an image map to the background input of the reflect node to cover those gaps where there's nothing to reflect. Then, go to render settings, and make sure that raytracing is checked. render, and that should give you a pretty good start.

If you want specular highlights in the mirror as wel (for example, if a light is meant to shine on the mirror and reflect into the camera), then make sure you have the specular value turned to something suitable. Otherwise, you can leave this black as well.

JonTheCelt