My quilt teacher taught me to use my ruler eeons ago. I use the little round sandpaper thingies you can get at JA's or Wal-Mart did carry them in the notion section. FOr my 5" x 24" i put one in each corner and then spaced two more per side. Been using this way for going on 4 -5 years without a problem.
But, more importantly is the way you place your hand or I should say, your fingertips. Spread your fingers apart and using your fingertips only, place on ruler, bring your blade in from just outside the ruler at an angle straightening as you cut a few inches, stop cutting, walk your fingertips up the ruler, continue cutting, doing so all the way up the ruler. When you are used to this method, then you can start walking your fingertips up the ruler at the same time you are cutting. Also, make sure your ruler botom and ruler top are on the same cutting line before you start cutting.
Too much pressure is placed on the ruler when you lay your hand flat and press down making a pushing motion when you least expect it.