also there is a clear product called Invisgrip that comes in a roll. it's sort of like cling wrap. you cut it to fit your ruler (just a tad smaller) and place on the back. it does help the slipping when you can't go out and buy new rulers.
also i love Omnigrip rulers but got out my 9 1/2 inch ruler and it didn't slip at all. i mean not at all. it's frosted and by Olfa, if you ever are in the market for a new one.