Andii, spray a little bit of basting spray on the back of your ruler. At first it will be very tacky and it will be a little trouble to position your fabric for cutting. But after a few cutting sessions, the tackiness will decrease, positioning fabric will be easier and your ruler will not slip. Before I tried this, my ruler was always slipping and, of course, my cutting suffered from my 'unkind' cuts. Since I sprayed the back of the ruler, my cutting has become much more accurate and my quilt tops show it!
You could probably get to the right amount of tackiness on the back of the ruler by gently rubbing some scraps over it and testing how well it holds on fabric afterward. Do that until the ruler has the amount of 'stick' to the fabric that you like.