Velcro, the sticky back kind, works very well to keep rulers from slipping. You can purchase the dots.
It's pretty dangerous to let a longarm ruler/template slip - easy to sew your hand to the quilt...and you can't reach the back to raise the needle either so you're stuck until someone comes home (which is why most of us keep a phone in our pocket when longarming!). So, it's common for LA'ers to use something on the templates/rulers. The velcro works very well. The only issue is that you cannot slide the ruler, which I also like to do when sewing fast - so some have it, and some don't.
One of the longarmers has recommended the skateboarding non-slip tape you can buy - she says it works great...and is easy to swipe a piece if you have skateboarders :) I find it pricey to buy if you're just wanting it for rulers.
Hope this helps some, at least in a pinch :)
Debbie in Austin