A couple of glamour composites with a mirror have already been shown. The first was posted on June 26, 2013 (The Go-To Model) and the second was from November 23, 2013 (Only With A Tripod - 1). This is totally easy to do if you have a sturdy tripod and a good model who can remember her poses. Simply affix the camera on a tripod, position it so there is a nice reflection in the mirror, adjust the camera to manual exposure, and take two shots of the model (in this case Laurel Rae Hervert) dressed two different ways but striking the same pose:
I used the Lasso tool to make a rough selection of Laurel's reflection in the second image. Next the selection was put on its own layer by Command-J (Control-J):
This layer was then Shift-dragged onto the first image. Holding down the Shift key aligns the layer into its original position. No feathering nor masks were needed. I then combined these two layers into one by Command-E (Control-E), did some minor retouching, then cropped and vignetted the image a little:
This is such an easy technique that I am surprised I do not see it more often. Perhaps it is because photographers dislike using tripods!