They started to buckle up because they didn't want to pay a fine. Today we do it because we all know that it's for our best. And when you seriously think about it, you'll agree that the possibility to lose their license is an important reason to not drive after a couple of drinks for most people, because they don't realize how the alcohol affects their perception already.
Another example would be smoking. The ban in public places changed the perception, few people keep complaining about their personal freedom being cut down, and in a generation or two this argument will be gone completely (like the one against buckling up: "how weak do you think i am, i can just hold myself").
It's proven how laws affect the behavior and thinking of the individual, and can even change morals over time. People like to act according to the norm, and the norm gets defined by laws a lot. On the other hand, laws that go boldly against the existing norm are not as effective as those which are agreed on already.
"Law can influence moral attitudes by recharacterizing behavior previously thought of as harmless"
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2292051
http://news.stanford.edu/news/2014/november/social-norms-jackson-112414.html
You can go far back in history and find more examples all the time. Even violence and self-justice have been a lot more common in early societies, until laws came up that frowned upon them.