In addition to all the excellent advice above, may I add my comments? If you're quilting on a domestic machine, the quilt sandwich gets handled a lot as it goes through the relatively small harp. This can destabilize your quilt sandwich, even if you have spray-basted. With large projects, after I have spray-basted (using the "board" method), I will machine baste a grid on my quilt sandwich with water-soluble thread. It's an additional step ... but it makes a big difference. Machine basting dissolves completely when you wash the quilt.