No, I don't mean lens glass. I'm talking about a situation that most people encounter rather often: shooting through window glass. Window glass can pose all sorts of lighting and reflection problems.
The above photo was taken 52-stories above Boston in the Prudential building's Skywalk Observatory. The combination of the beautiful low-light dusk exterior view and brightly lit interior of the building created all sorts of reflections on the window glass that I was shooting through. Even with my camera on a tripod and the lens right up against the glass, it was still possible for light to sneak in around the edges and reflect off the window. These sorts of reflections usually show up in photos as bright, unpleasant blotches which reduce contrast and are extremely distracting.
How did I get such a clear photo? After framing the image and focusing my camera, I used my black jacket to cover the camera and separate it (along with the window glass immediately around it) from the light sources behind me. The color of the jacket (black) is important here. Lighter colors tend to reflect more of the exterior light which just bounces back off the glass and into your camera. This involves a little bit of preparation and forethought. When I anticipate taking a photo like this, I take along either a black cloth or wear something black that I can take off and use as a light shield.
This simple technique can help you get much clearer, sharper and overall better photos... whether you're photographing a cityscape from inside a brightly-lit building, or just taking a snapshot through an airplane window.