BFF and I were talking about how much we love rotary cutters as opposed to scissors. However, we both get frustrated because the ruler that you're using to cut a straight line tends to slip, and there goes your straight line.
BFF told be that her mom has a ruler with a non-slip grip. Then a light bulb went off. Hot glue tends to have the same effect. So I glued a thin layer of hot glue around the edge of my ruler and, voila! No more ruler slip!
I'm sure many other people have already thought of this, but I thought I'd throw it out there just in case.