There are many staple techniques for creative portraiture, and one of my favorites is shooting through glass and reflections.
Many of the portrait techniques I've grown to love over the years are either heavily inspired by, or an outright copy of techniques used in high end street photography. I love street photography's use of contrast and light, but I also love the many shots of people through windows. It is usually in a coffee shop somewhere in the city, but any glass that hosts reflections will do. There have been entire photo series based on this premise.
In this video, Matt Granger takes you behind the scenes on setting up a photoshoot with the express intention of using reflections for interest in the images. This can truly be done using any glass, and you can use water or lights to add the interest you desire. One tip I would give is to use manual focus if you don't want your camera hunting and missing focus regularly. Even with Eye AF, it's easy for the camera to get confused and latch onto a water droplet or reflection on the glass.