If i get what you are trying to do right - it is going to be an extremely difficult thing to get looking real... but not too hard to actually get what you want.

You need to get the shots of the model on separate layers, the "reflection" layer on top... line them up then slightly reduce the size of the "reflection" and move it into place where you want the mirror to be behind her (you might want to have the layer slightly transparent so you can see the "real" layer beneath)... if you need to have a play around with perspectives then just open up the tool under edit>transformations.  Once you have the right look to the reflection, all you need to do is to mask out all of the reflection layer which isnt visible to the eye - ie mask out everything where the "real" shot of the model is... that will put the reflection behind the model and voila you have a reflection behind her... not sure about a mirror but you at least have a reflection.

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography