When it comes to glass it's all about lighting direction.

You want to make sure that when you look at the picture through the camera neither the reflection of the lightsource or anything lit by your lightsource is visible.

Hold up, I'll draw a diagram: //

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I happen to frequently photograph artwork, including framed and with glass.

If possible, do this in a room that has black walls and no windows. If such room is not accesible, wich is my case, I use a black backdrop BEHIND the camera. This helps a lot against unwanted reflections. Also, as the other answers point out, place the lights a 45 degree or more relative to the line from the lens to the center of the artwork. It means, 2 simetrical lamps, one at each side and at the same distance from the front and from the sides of the artwork.

For example, if dealing with a 1 meter wide painting, lamps would be rougly 1 meter in front of the painting and 1 meter away from the edge of the painting. Lamp heads would be at the height of the horizontal centerline of the piece. Partly close the barn doors to limit light output so it falls only on the painting.

I Use a tripod and fire the camera using a computer and a usb cable. This is to avoid unwanted movement of the camera and to avoid my own reflection on the glass. ....

When framing, the lens axis should point perdendicular to the center of the artwork (specially if it's rectangular) to avoid perspective distortion. (It's correctable in post, but preferable not having to...) //

This works well but I have, in extreme situations, used a black drop cloth with a hole cut in it for the camera lens so the camera, and photographer, are behind the cloth.