Depending on the size of the ruler, I just dab several spots of clear nail polish on the back of the ruler, i.e., on a 24" x 6" ruler, I probably have 8 dabs of polish, 6" square, 4 dabs. Then I just sprinkle w/salt. Wait a few minutes for the salt to adhere and then dump the remainder in the trash. -- I must of done this 6 months ago and the salt is still holding. Will repeat at the same spots when needed.