Flat on, with even, diffuse lighting - unless it's a sculpture or using a painting (etc.) techique where shadows would add to the texture. A tripod would probably be helpful.
And, of course, after having secured the permission of the artist/copyright holder.
You will need to use 2 light sources from both sides of the artwork.
This will create a flat lighting and will create a perfect copy of your subject.
If your subject is behind glass, you will need to play with the angles no to get a reflection of the light source in it. If this is the case try using longer focal length lens, and move away from your subject.
Besides having even lighting from both sides (I find as low as 30° to reduce glare), you need to have the artwork and the sensor plane abstlutely parallel, and the camera centered on the artwork -- use a trplod and move it up or down if the art is on the wall.
If you are using a zoom lens, try to find the zoom level with the least distortion, and then move in or back to have the art fill the frame as much as possible; you can pull back a little if you have a large sensor and/or want to include a color chart.
Once you are set up, you should set a custom White Balance using a white/gray card under the lighting you'll use.
Use the timer, check the focus, take more than one shot of each piece....
The actual tool used for this kind of work (for smaller pieces, anyway) is called a Copy Stand -- it looks kind of like an old-fashioned enlarger with the art on the base and the camera where the negative would sit.
