I sometimes don't have enough backing material so what I like to do (and it adds interest to the back) is insert another color, pattern, or whatever may coordinate with the front and back. If the backing is too short, I cut towards the bottom, maybe leaving a 12" piece or less depending on how big the quilt is, and add a strip of the coordinating fabric wide enough to make the backing long enough. Just be sure to leave enough at the bottom of the original fabric so you can see the insert. You can do the same thing if the backing material is not wide enough. I posted a D9P in the Pictures section that I recently finished that I inserted a strip of the front D9P fabric to the back because my backing material was not long enough. Modern style quilts are usually pieced like this. There are some great examples of this on Flickr.