This tip applies mainly to all you 2d artists out there.
When you photograph your artwork, make sure the sides are in parallel. Sometimes paintings and other 2d images will look warped or fish-eyed in a photograph.
If you work is framed and the frame is not integral to the piece, consider photographing it unframed. Frames can present several problems: they often contribute the out of parallel effect, works under glass often have glares or reflections, and frames take up valuable real estate in your image that could be utilized by your actual artwork.
If you need to, it is often better to sacrifice a small sliver of your painting rather than to have a distracting slice of background in the frame.
Photo-editing software can help too. Many digital cameras come with scaled down version of software such as Photoshop elements. Additionally, there are several online services such as Picasa and Photoshop that offer basic editing tools like cropping for free. More complex programs such as Adobe Photoshop give you options to stretch or warp your images back into square, but they must be used with discretion so as not to alter your artwork too much.