The biggest one I've seen with my own eyes is smoking outside in public. When I first came to Japan you would see lots of people smoking while walking, or smoking on outside train platforms. Then one day somebody put up a big billboard all over the place that had a drawing of a person holding a cigarette and a child. It said, "You hold a cigarette at the height of a child's eye" or something to that effect. From that day on, practically everyone stopped smoking and walking. It's really incredibly rare to see it now. It might have been a localised effect around where I lived, but other people I've talked to from around the country remarked on it as well.
I've seen other things like it, but this was the most remarkable to me. The government seems to randomly put up signs like that to see if they take hold. It never seems to go gradually -- with early adopters, etc, etc. Either everybody goes with it or nobody does.
I'm trying to think of something that is easy to verify, but it's nearly midnight here and I can't think of anything off hand. If I remember I'll try to post back later.