I've done a couple of different things. On one log cabin, I made the backing one giant log cabin block. When I have leftover blocks (or material to make a few more) from the quilt top, I make a strip of those (either horizontal or vertical) to connect two smaller widths of fabric together. I have even taken odd-sized leftovers and just pieced until I had a large enough piece for backing. I buy wide backing, but only when its a deal too good to pass up.