HST are tough for me to keep square, too. I make them by adding a full inch to the finished size I need. Put the two squares together, draw a diagonal line and sew 1/4" from each side of the line. After sewing, cut them apart on the line. They are then a touch oversize so I have room to square them up.
It's a bit more time consuming, but it's the only way I end up with squares.