I recently had surprisingly good luck photographing a glassed piece of art. Don't know if this would work for everyone.
It
was dusk, or nighttime. I laid the art on the floor on a white sheet
of plastic. The ceiling above was plain off-white. I turned off all
the lights in the room. I set my Cascio digital point-and-shoot camera
on "no flash," stood over the painting, centered it, and shot. There
was a small flash from the light meter. The resulting photo had NO
reflections! Color was GREAT!