You've already gotten a lot of great suggestions and I've tried most of them, but I want to throw one more out there. Using a hand weight on my ruler was a game changer for me, but my best go-to non-slip trick comes from Bonnie Hunter - I use NexCare Transpore transparent surgical tape on the back of all of my rulers and templates. It works great, you can see through it, and it's easy to remove.