I had always had problems with my HST's also until I started doing the method that BarbC just told you about. Its a little more work than just cutting and sewing and hoping for the best. But the results are very rewarding. When squaring up and cutting down to size be sure and put your block on an angle mark and cut down gradually. But other than thats its very simple. Hope this helps you as HST's are fun and really dress up a quilt.
Rita