You gotta hang a sopping wet washcloth on it.
Lately when I go to drain the bath water, the flippy, water stopper thing gradually eases its way from the "down" position, to the "up" position. It originally kind of freaked me out, like some supernatural force was playing a game with me. But after mentioning it to a co-worker, he said something along the lines of, "Sounds like you got some hair and gunk cloggin up the pipe, causing some pressure..." But instead of calling my manager, and risking the humiliation of whatever "gunk" might be in there, I have discovered that draping a heavy, wet washcloth over the flipper does the trick. This weighs it down and keeps it from snapping up (and keeps me from discovering nasty, night-old bath water the next morning when I go to shower).