In the airlines I fly with there's always an announcement advising the pax to keep their seatbelts on during the flight even when the 'fasten seatbelt' light is turned off. The majority of the pax don't seem to give a toss about it. Then they complain they've been tossed about.
Seems like such cases of injured pax is a more effective way of getting the message across albeit it being a painful one.
*rant over*