How to fix a loose rear view mirror base? Any ideas??

I've done this a bunch of times. Remove it from the car and take the 3 screws off the plate that tightens the ball on the inside. Then cut a little square off a cereal box and place it in between the plate and the ball and tighten the screws back. It'll be tighter than a you know what once you get it back installed. It might be a good idea to wire wheel the mirror arm while it's off if it has any pitting or rust.