One thing that helps me is heavily starching both the top and batting before sandwiching. Starch stabilizes the fabrics so they are less likely to stretch as you quilt. It is the subtle stretching of fabric as you sew that causes the excess fabric to bunch up against the stitching line you are trying to cross. For the backing I like to use a 1:1 solution of Sta-Flo and water. For the top I use several layers of spray starch. You can even add layers of spray starch after sandwiching and it will help.