Photographing through Glass
Photographing through a sheet of glass is supposedly one of the hardest things to do in photography, but has one of the easiest solutions.
The problem is that glass is reflective, even when it's much lighter on the far side of the glass than where you are photographing from. The result is that not only does your reflection show in the picture, but both metering and autofocus can be fooled by the reflection. Worse yet, if you are using a flash to illuminate your shot, all you will see is the reflected flare of the flash.
The solution is surprisingly simple. Hold your camera with the front of the lens flat against the glass you are shooting through. All reflections will be cut out by the barrel of the lens itself. However, be careful, as the barrel of the lens may rotate as you focus, and if you have it held too firmly against the glass, this rotation could possibly damage both the lens and the glass. Therefore, it's a good idea to focus with the lens held very slightly away from the glass and then press against the window before fine tuning your focus. Using this method, even a flash won't show up as a reflection.
This works fine if the object you are photographing through glass is dead ahead of you, but what if you need to photograph through the glass at an angle ? Well, the same principle applies, you just need to find a method to block extraneous light that can get in between the lens and the glass. If you have a rubberised lens hood, that may be flexible enough to do the job. If not, I find cupping your hand around the end of the lens to act as a lens shade between the glass and the lens usually works well.
Exposure through glass is quite straight forward. If you use the methods outlined above, then your camera's metering system should provide satisfactory results. However, things get a bit more complicated when using flash, particularly if shooting through glass into water (e.g. an aquarium). The problem there is that the water reflects the flash light in unpredictable ways, especially if you are shooting just below the surface of the water. the water acts as a reflector and bounces the light down into the area you are photographing, over exposing the subject of your picture. There are two ways around this: where possible, shoot as far from the surface of the water (or any other reflective surface, such as another side of the tank) as possible, or, if your camera allows, underexpose your flash shot by one or two stops.
Try it...your local aquarium is a source of many great and unusual shots.
Sample Shots
Both these photos were taken through glass...