By far the best thing you can do to make great photos of your artwork is to carefully choose your light.
How do you choose your light?!?? you ask. Simply override the automatic, flash-everything-to-a-nice-pasty-white function on your camera. Shoot outside on an overcast day or utilizing indirect light, i.e. shade.
Turn off your flash
Sometimes you may want to try adding extra light. Your situation - your art media, your camera, your weather - may make it so that pictures shot with a flash are more accurate.
In the following example, the flashed picture is not too far off from the original. The non-flashed image is a bit oversaturated but still has more color and detail than the slightly washed-out image. Either of these images, with a little help from Photoshop, can be made more accurate. Without going digital, I would opt for the richer, more colorful image than the flashed image. You will especially note the difference if you click on each image to see the enlargement.