I've found that many people sit down while they are using their rotary cutter, or even try to cut from side to side. I can only suggest you stand and use the full weight of your shoulder going down to the ruler and cut from your body to the top of the cutting board. Also as someone else suggested 'spiderweb' your fingers on the ruler, instead of leaving your hand flat with the weight of your shoulder only going to your wrist.
Hope this helps.