I have had excellent results with Invisi-Grip, and agree that Creative Grids rulers are so good they need no assistance to prevent slipping. However, I am surprised no one has mentioned the following solution to slipping or moving rulers: cut small pieces of rubber shelving sheeting and position two of them under different parts of the ruler between the ruler and cutting mat (NOT on the fabric being cut). Pressing firmly as usual keeps the ruler from slipping as it adheres to the cutting mat and is flush with the fabric being cut. Keep these rubber shelving scraps everywhere -- on your cutting mat, in your traveling tools bag, with your machine, and you will always have grips under your rulers when you need to cut fabric.
Karen