I got a BlocLoc HST ruler at the Grand Rapids quilt show last summer, and it is WONDERFUL. I cut a square of fabric about 1" larger than the finished size of my HST. I draw a line along the diagonal, then sew a very scant 1/4" on each side. Cut on the line, then square up using the ruler. I tried all kinds of other square up rulers and techniques and this is the thing that really works for me. With the other methods, I'd wind up with a square the right size, but could not get the center seam to consistenly wind up smack on the corners. I just did a small quilt, 64 HST, ALL my points matched without even trying!
They come in lots of sizes, but you really just need the 6" one. There are videos on the site.
http://www.blocloc.com/blocloc.htm