I heavily starch the backing before sandwiching. (Actually, I heavily starch the yardage before even creating the backing, but you can do it to the backing also.) My method is fast and easy. I mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this solution onto the yardage until it is saturated, toss in dryer, then iron with steam.
What this heavy starching does is stabilize the backing fabric so it doesn't stretch or distort while you are machine quilting. In my experience, it pretty much eliminates puckering. For the same reason, I starch the top before sandwiching although in that case I use layers of spray starch.
For a quilt that is already sandwiched, spray starching both top and backing several times will add some stability and often will stop puckers if you are already having that problem.
Spray basting also helps prevent puckering because it holds all surfaces together at all points. Pins, for example, hold the sandwich together only every 4 inches or so where the pin intersects the sandwich.