One of the simplest ways to eliminate glare from a flat reflective object, shooting straight from the front, is to get a large piece of matte black foam core and cut a round hole in the middle the exact size of the lens. Hang it on the lens close to the camera body, center the subject and shoot. Be exact with the placement and size of the hole for good balance of the board.
It's important that the board part facing the subject is black, not white, as white will bleach the color out, and reflect the black lens ring. The black also renders the contrast and color of the subject better.
I shoot glass-faced clocks as a product photographer, and this is really the best, the cheapest, and most time-efficient way to eliminate glare and reflections on flat faces. I actually hang a large 4' x 4'piece of black-painted coroplast from the lens for larger objects like our 35-inchers. The camera frame can easily handle the extra weight of the boards since they are designed to hold very heavy lenses.
Cheers, hope this helps