I have an idea for a short that I'd very much like to work a plot around and use.
It focuses on the idea that a persons reflection shows another side to them. I'm just wondering if anyone knows some camera tricks I can use that can enhance that kind of feeling or give across that kind of atmosphere?

You could tint the mirror a bit to recolor the reflection... fun house mirror types of stuff. Use different framings/handheld vs. tripod or steadicam when shooting the reflection as the reverse on the actor.