I always heavily starch my backing fabric before layering. To do this, I mix a half:half solution of Sta-Flo liquid laundry starch (available in laundry section of grocery stores) and water. I lay the fabric out on my kitchen island and use a large house painting brush to "paint" the fabric with starch. Once it is saturated, I throw it in the dryer to dry, and then iron it with steam. It comes out pretty stiff, almost like thin cardboard.
It's also a good idea to either starch the fabrics for the top before cutting (you can use a weaker starch solution for this) or spray starch the top before layering. (Be careful with spray starch. It needs to soak into the fabric before you iron; otherwise you can scorch the starch with a hot iron. Experiment before using it on a quilt top!)
The starched backing fabric will not pucker and wrinkle when you free-motion quilt. Just be aware that spray basting does not stick as well to starch as it does to fabric. If you are accustomed to spray basting, you may want to supplement it with some rows of machine basting with water-soluble thread.