I read in one of my old quilting books to use clear nail polish and then sprinkle it with coarse salt. I don't recommend this!!! But I thought it was interesting to see what was used back in the pioneering days of rulers and rotary cutters.
That being said, I use Odif 404 ruler grip spray. I got it on closeout from Walmart so I don't know if it's been replaced by the stuff QuiltinNan uses or it has just been relabeled. This stuff ends up covered with threads and fuzz eventually, but it also washes off easily.
I have also used the little sandpaper dots. They work well, but are not see thru.