I learned this tip from another forum several months ago. Go to the pharmacy area of your favorite store and in the band-aid section buy a roll of NexCare brand Flexible Clear tape - 3/4 in. X 252 in. for less than $2.00. Put strips on the underside of your rulers, I put one on each of the four sides about 1/2 from the edge, you can see fabric thru it and it does not slip easily. I really like how this works. One roll did all my rulers.
Ruth