I generally use a fabric that works with the top. Sometimes I luck out and there's a wide back fabric that works. Sometimes I piece a back using blocks left over from the top or using fabric left over from making the top. Most of the time I'm able to find something that will work that is already in my stash, though there have been a couple of times that I purchased fabric specifically for a back, and several times I've over-dyed fabric that I had in my stash in order to change it to a color that works better with the top.
Rob