With only trial and error as your method of shooting, glass can be a big pain. However, a few simple tips can make your job a lot easier.
First, turn off our flash (you still have that thing on?!?). Next, possibly enlist the help of a filter. Third, watch out for reflections.
Glass, Plexiglas, Non-glared Glass or No Glass
Again the best thing is to turn off the flash. With the flash on, you will need to place yourself at a 45 degree angle and this will be a strict violation of rule #3, Keeping Things Straight.
You may begin to think we have a thing against flash. We don't. In fact, we love it. But only in certain circumstances. As an artist, you want the best and you probably have a keen eye. In this case, using a flash against glass will not give you want you are after; instead your painting will be obscured by a big ugly glob of glare.
The only other thing you will want to do to make a good exposure of a painting behind glass is to watch out for reflections. It sounds simple but it is easy to completely overlook these at the time you take the picture. To combat this, take a moment and look everything over carefully before shooting. Make a note on your hand if you have to! Tape a little piece of paper to your sleeve that says, "Wait! Look! What am I going to see later that I am overlooking now?" And then adjust yourself, your camera, or your painting until you get it right.
French Church, Just Right