I went to a workshop with Pat Speth and she gave us each a little (3 inch) piece of the rubber shelf liner to put under our rulers. Works like a charm. (However, I now have 2 triangles....can you guess?)
Another thing that I have discovered...I had a ruler with a lip on it to use like a "T" square. It wasn't square. It would be off by 1/8 inch every 18 inches. Not good. Now I use a metal ruler that I can line up on my cutting board all the way across. It is a life saver. Lots more accurate than the first ruler. Hope this helps.