Spray baste and a walking foot... it made all the differnce in the world to my SID.
Once I get the backing and the backing and the batting layer together I take a cool iron( no steam) and smooth it all out backing side up. This also helps the spray baste to grip all of the fabric. I repeat then with the top. I have been pucker free since I started doing this.
I do still but some pins on the perimeter just to keep it from lifting with all the repositioning.