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:

Glass and other shiny objects reflect light back in one direction (like a ball bouncing off a wall). The painting, which is diffuse reflects some light this way, and reflects some light back in every direction.

In the setup above, the light from a directional source hits the glass and carries on, missing the camera, thus the glass isn't visible! The same light hits the painting and some of it does get reflected into the camera so the painting does show up.

The important thing is to use a directional source, as it's possible for light from your lightsource to bounce of a white wall, and hit the glass at an angle that does go down the lens, and this shows up as flare.

This often happens when you have a white wall behind the camera, as in the above example.

The moral of the story is to make sure you can't see the reflection of the lightsource in the glass when stood behind the camera, and try to limit what the light hits (ideally it should hit nothing but the painting). You can make any lightsource directional by placing opaque objects around it to block the light in certain directions.