There are several approaches to composing a photograph of your artwork.
For jury slides, a simple compostion is probably best- center your work in the frame. Two dimensional work should be cropped to show only the work. Unusually shaped and three dimensional artwork should fill as much of the frame as possible.
For advertisements or other venues you may want to create a more interesting or dramatic compostion.
Either way, be wary of odd or unflattering angles. Really look at the framed image in your camera lens as if it is a piece of art itself.