Sandpaper dots every time!
Just make sure you dont place them on frequently used measurements (ask me how I know ...
)
Omnigrid do rulers with non slip already attached ... Omnigrip
Now Im not suggesting you need buy new rulers but maybe look closely as to where the non slip discs are placed ??!
There is a plastic film (like you used to attach tax discs with) available too but I found that impaired the visibilty ...
But that could be just me!
If you want to try that tho' let me know as I have a sheet of it somewhere that I will gladly send to you.