How are you making your HST's? I find it easiest to cut two squares 1 inch larger than the finished size I want. Draw a diagonal line on on square. Sew 1/4 inch on each side of the line, cut apart and square up.
Here is a link that explains it well. The instructions say 7/8 inch larger but I do 1 inch. I refuse to measure 1/8!
http://quilting.about.com/od/quickpi...squaretria.htm