We had that problem in a kitchen we are currently remodeling. Part of it fell in when we were demolishing so we had to take a serious look at the rest of it. What we did was to feel around the other areas that were sagging. If they felt loose, we'd rip them down and patch them with dry wall. Where they still secure, we carefully screwed the plaster tighter to the ceiling and painted it and splotched it to match the rest of the ceiling. SOmeone told me once that once the plaster starts to pull away from the lath, it's just a matter of time. Eventually it will fall in-maybe decades later-but it will happen. Good luck with your project!