I starch both the top and the backing. ;) I pin across one row, then move it all up onto my dining table, and finish pinning, frequently checking that it's all still lying flat.
I also mostly free motion quilt using the Flynn type frame my DH made me, and I think that also helps hold everything more taut.
I'll be stitching in the ditch on my son's flannel log cabin prety soon, just using my walking foot, no frame, and will have my fingers crossed... lol!!