Yup, starch! It stabilizes the backing fabric so it doesn't stretch and pucker on you while you are quilting. I heavily starch my backing fabrics before layering.
When you already have the quilt sandwich put together, try misting the back with spray starch several times, allowing it to dry in-between. Several people have said that stopped their puckering problem.
Oh, and I do think that spray basting helps also. Spray basting is the only kind of basting I do now; it's *so* much faster than other methods.