The best investment I ever made was Quilt in a Day triangle square up ruler. It saves so much time and cutting and your HST's come out perfect every time with only ONE cut.
The way it works is before you open your sewn HST you lay the ruler for the size square you want on the sewing line and line up with one edge and cut the other edge. Done.
The ruler also comes with instructions for a quick method to make 8 HST's at a time. These are slightly oversized so you only trim a sliver.
http://quiltinaday.com
You can watch videos on useing her rulers.