One of the biggest challenges in photographing artwork is eliminating glare. This is less of a problem when photographing matte surfaced work like watercolor paintings, but can be a real problem with work that has an inherent sheen.
There are several strategies that can be used to eliminate glare. If you are using artificial light, you can adjust the positions of your lights and camera to reduce the likelihood of glare becoming a problem. Placing your lights further from the artwork, placing your lights at a shallow angle to the artwork, and moving your camera further from the artwork, all help eliminate glare.
Placing a polarizing filter on the lens of your camera may help. You probably remember from high school science class that light is a wave form of energy. In the light emitted from the sun or a light bulb, the waves are unaligned and the axis of vibration can be in any direction. Light reflecting off a surface on the other hand, tends to be vibrating in a narrow range of directions–it is partially aligned or polarized.
A polarizing filter similarly tends to let light waves pass through that are oriented in one direction while blocking light waves that are oriented at a ninety degree angle. When you rotate the polarizing filter on your lens, you can block the partially polarized light from you art work that is causing the glare. But since the light from the glare and the light passing through the filter are both only partially polarized, you still may not be able to eliminate the glare completely.
When these simple strategies fail, there is a fail safe method: cross polarization. With this strategy you place a polarizing gel over your light source and then use the polarizing filter on your lens to completely eliminate glare. You can see the impact in the samples below. Since both the polarizing gel and the polarizing filter eliminate a portion of the light passing through them, this strategy does require longer exposures. Also, it is important to create custom color profiles from your setup since neither the gel nor the filter will be perfectly neutral in color balance.