I heavily starch the backing fabric before layering and spray basting. I have access to some large tables for spray basting. I lay out the backing fabric, then use large (easy-to-use) plastic hand clamps I got from the hardware store to secure the backing edges to the table.
Next step is to smooth out the batting over the backing. Once I'm satisfied I've got it on straight, I pull down half the batting, spray the batting, then push it back onto the backing. Once that is all smooth, I turn up the other half of the batting and repeat.
The top goes on the same way -- position first, then pull down half, spray, smooth, pull down other half, spray, smooth.
This method works even if your table is smaller than your quilt; you just work in sections. The clamps are what keep everything in place so no unwanted shifting occurs.
If the quilt back is too large for the table, you can still spray baste in sections using this method. The clamps help keep everything straight.