When you have your ruler positioned, lay your non cutting hand flat on the ruler but with your little finger (or little and ring fingers) on your cutting mat. It helps keep the ruler steady. If you are cutting more than 12 inches, either stop and reposition your hand, or use a weight towards the end of the ruler (a great way to get a little strength training in)
. I use a 5 pound weight at the end of the ruler. It really helps on those long cuts.
A sharp blade cuts much cleaner than a dull blade. I know they are expensive but oh, what a difference!
And of course, measure twice, cut once.