I place my non-cutting hand at the far edge of the ruler I'm using, with my thumb and pinkie spread and resting on the fabric so the ruler doesn't move. If you find yourself pushing down on the rotary blade to cut, it's probably time to change your blade.
I agree with those who say you need to have a plan in case something happens and you need help. My friend Cindi once sewed through her finger on her longarm. She couldn't reach the power switch or her cell phone, and her husband was downstairs at the other end of the house in the garage. She was stuck for hours until he finally went looking for her. Make a plan, keep your cell phone in your pocket.