I recently experimented on one of my portrait shoots with photographing through a glass door. I wanted to see how I could incorporate some kind of reflections into my portraits to add a bit more of an interesting layer of detail. I found that the angle at which I shot became even more prevalent as the reflections differed so massively from one viewpoint to the next. I think when doing this kind of photography, it is important to think about how much you want your subject to be the main focus and how much detail you want to gain from your reflections. The examples below show how I have used one window and one subject at the same time on the same day and created three completely different images.
The sky can be a really effective reflection in this scenario, especially if you have some interesting or dramatic clouds. On a grey cloudy day, you can use the reflection to create a foggy effect like I have above. You can use this effect to create more of an abstract image depending on your style.
When photographing through windows it's important to consider what's in the background behind your subject that you might be able to see in the frame. Keeping the background as minimal as possible means that there's more focus on your subject and the interesting reflections without the image becoming too busy.
Using the curtain from the window can help to give you a plain background to work with. However, you may want to be careful that your subject doesn't look too much as though they're being a mannequin in a shop window.
Remember that shooting through glass will make your image less sharp, and could be harder to get your subject in focus. Have fun and experiment!