I heavily starch my backing fabrics before layering to prevent this problem.
Since your quilt is already sandwiched, my suggestion is to spray starch the backing several times. Let the starch dry between sprayings; I do not think it would be necessary to iron. I recently gave this advice to someone else with the same problem, and she posted that it worked for her.
Starching stabilizes the backing fabric so it doesn't stretch as you sew.
If you try this technique and it works, please post back! (Also post back if it doesn't work. :( )
I think it is very difficult to correct puckers later. It is much better to prevent them, and starching helps with that.
Be careful of overspray with the spray starch. It would be best to lay down several large sheets underneath your quilt to catch any overspray, and try to spray from the edge of the quilt towards the center.