In addition to SITD, you could stitch diagonal lines across the quilt, or horizontal and vertical lines, not on the block seams. If you don't want to hand or machine quilt, then you can tie. As mentioned there are lots of good tutorials and videos out there. If your machine has a built in bar tack, you could also use that instead of hand tying. Whatever you do, you need to read the info on your batting about how closely you need to secure the batt so that it doesn't come apart. Remember the recommended distance is in all directions, including the diagonal.