I think airlines should emphasise a little stronger how important it is to keep buckled up.
People who feel that seat belts are important usually put them on without prompting. People who think seat belts are too uncool to wear usually aren't going to grasp their importance from seeing a safety demonstration, no matter how many times it's repeated.
It's true for airplanes, cars, roller-coasters, etc. Some people are safety-conscious and some aren't.
I haven't seen exact numbers, but my guess is that the difference in the number of people who belt up when given basic safety information and the number who do so when given an exhaustive, strong lecture on safety isn't that big. If they don't get it the first time, they never will.