In another recent thread someone suggested using rubber cement. I really like that idea because you can rub it off if you don't want -- so I could just grip the part I want to grip.
One of the ladies in my quilting group is big on gripping rulers. She has used the spray and likes the tape better.
For me, sometimes I want the slide and sometimes I want the grip and I don't want two of the same sized rulers, so I'm thinking rubber cement may be the way to go.