Lufthansa has a very easy solution. When seated you are REQUIRED to have the seat belt on, even when the sign is off.
Since it is part of the PA right after take-off, along with the non-smoking message it makes it a legal requirement...
As for myself, seat belts are on when seated. And I go my wife to do it as well. First she did not understand until we hit a moderate turbulence over the alps. Since then she is convinced that her (aviation working) husband actually knows what he is talking about!