I find that my ruler tends to slip when I don't have good leverage. Standing over the ruler works best for me. I also do not try to do a full range cut in one motion. Instead, I firmly hold the lower half of the ruler and cut about half way, then I walk my fingers up to hold the upper half and cut the upper half. For really long cuts, I even distribute the weight into thirds.
When I need 4 strips @2.5" each, I cut one strip at 4x2.5=10" I trim the 10" in half and each 5" in half again. This seems to help with getting wonkier as I go.