Spray basting helps. Starching also helps a *lot*.
I heavily starch the backing fabric before sandwiching. My method for yardage is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" it on using a large wall painting brush until the fabric is saturated, toss in dryer, then iron with steam.
I also starch the top, but for that I use spray starch.
Starching stabilizes the fabric so it doesn't distort while you are working with it. Spray basting helps keep all the layers together so one fabric doesn't stretch more than the other.
When sandwiching, it's important not to stretch the backing. You want it lying perfectly flat, but not stretched tight like a drum. If you clamp or tape a stretched backing down, once you unclamp or untape the fabric will spring back to its normal tension, creating excess in the top and batting.