Use the classic method. Lay your opposing pinky along the edge of the ruler on the opposite side from where you are cutting. Press straight down with your rotary cutter instead of pressing against the edge.
I bought Grip Strips to help with this issue and I found they made the ruler up too high and I would get an incorrect cut. btw I also bought their gadget to make two rulers stick together to make a right angle cut -- they had the suction cups upside down. I kept wondering what I did wrong, lol!
When I made a pinwheel tree skirt I followed the advice to put packing tape on the back of the ruler, that worked! Apparently the edges of it were sticky enough to keep the ruler in place. Good thing since it was a homemade ruler.