I usually tear my backing fabric to the correct length. Then I tear that piece in two, slightly off center and insert a strip of contrasting fabric to re-join them to get the correct width. It offers some visual interest on the back. I've also done it the way Maniac2 does by using some orphan blocks in the backing.
The diagonal calculator looks useful if you have a space big enough to lay your fabric out so you can cut it on the diagonal.
Rob