is the pattress back box loose ???????or the switch cover screw threads worn ?????????
power off, remove the switch front, disconnect the wiring with a battery drill make some fresh new holes through the metal pattress box,change to a masonry, and drill 2 holes and then rawl plug the new holes, screw the pattress up, then refit wiring and the switch front .
if the cover screws are you problem follow the other answer
wirecutterman, June 2011