I do what Bonnie Hunter suggests, I make the backing from 10 inch finished squares. It's easy to calculate how many you need and you can get 4 squares from a width. I try to use fabrics that relate to the front of the quilt, any leftover fabric or extra blocks, but I also try to use up some of those "what was I thinking" fabrics. A bonus is that after you've trimmed up the layered sandwich, you should have strips that are perfect for a pieced binding.