If you heavily starch the backing, that should help keep it unbunched while you pin. I use a 1:1 solution of Sta Flo liquid laundry starch and water.
Have you considered spray basting? With that method you can put the batting down first, then smooth the backing over until you are satisfied. Flip so other side of batting is up and do the same with the top. This makes it easier to get both sides the way you want them.