When I am traveling as a passenger, I usually ask people to put it away properly. I certainly helps that most of my flights as a passenger are commuting to and from work, so I am in my uniform--instant credibility, I suppose.

If the bag doesn't get stowed properly, and I am not in uniform, I would probably flag down the FA as she walks by, saying something like "Ma'am...looks like some help is needed in finding a place for this bag..." or something like that would sound courteous and wouldn't hammer the doofus (oops, did *I* say that??) for not listening/knowing/caring.