Bolt came loose on drain

I have a (I think) Mountain brand tub drain/overflow kit. It uses a cable operated knob to raise the stopper. The brass chromed stopper fits through a hollow stainless steel bolt, which is what holds the stainless steel drain cover to the shoe on the bottom of the tub. There is a rubber u-shaped ring that provides the seal between this strainer on the top and the shoe on the bottom of the tub flange and the tub's drain hole. This bolt came loose, allowing the thing to leak for awhile before it was noticeable from downstairs. Got it tightened back up and it doesn't leak.

Any suggestions on what I might put on the bolt threads to prevent the thing from loosening up again. I don't want to have to repair the ceiling again!