Until just recently, whenever someone talked about pictures of a person taken in a mirror, I only associated this kind of pictures with Facebook profile pictures of teenagers (I think you know about what kind of picture I talk about). But I have discovered the great advantage of mirrors during my recent photo shoot in Poland - mirrors are great to frame a subject. There are other ways to frame a subject, for example with door frames, windows, trees, etc. but I never used a mirror to frame a subject before.

Using a mirror to frame a subject can be a bit tricky and difficult when you use strobes and/or speedlights because these additional light sources can create reflections in a mirror and light spots. Therefore, you should be careful when using lights for these kind of shots - a quick tip: first get your model or subject in a right position and take a test pictures to see what is visible in a mirror, then position your lights so that they are not visible in a mirror and are out of the mirror's frame.

Another thing is that any lines (within mirror's frame or any other lines) can be used to define a picture or a subject. Lines or even patterns can be very helpful in drawing the attention of a viewer to the subject in the picture or separate the subject from the rest of what is in a frame. Next time you'll be taking pictures, take a close look at any patterns or lines either in the foreground or in the background, try to change your position or reposition your subject to use these lines.