the file label, or a similar sticky back paper will work. some scopes come with "friction pads". sticky back labels will do the same thing.
i usually put them in both bottom and top ring halves.
my ruger redhawk 44mag with factory rings has been about the only slipping trouble i've had. slipping stopped imediately after adding friction pads.
may also look into reaming/lapping scope rings for proper alignment. i recently made a tool for lapping. 1" solid bar stock, drilled/tapped on the end for a long tap screw. you can then turn the bar with a drill to even up the rings internally. it also works well for turning in dovetail rings. lapping kits, or reamers can also be purchased. possibly "brownells", or "sinclair intl".