Several people here recommend really seriously starching the backing fabric. Some do spray starch and some do liquid. Go to "Search" at the top of the page, put in "Starch" and you'll get lots of information. Both for cutting accurate pieces and getting a smooth backing, lots of people starch.
Someone recently said on one thread that you can add spray starch to your backing even if you have it already basted to your quilt. I've never tried it myself but I thought it was an interesting suggestion.