I use the grip strips and my ruler NEVER slips anymore. I used to have problems all the time with slipping but since I put grip strips on all my rulers I never have any problems anymore. Nice thing is you can cut them to any size to fit any ruler, half hexie, 60 degree etc. I'm not affiliated with the company I'm just really thankful for their product. And the price is very reasonable.
http://www.guidelines4quilting.com/GripStrips.html