The rapid decline in smoking among the U.S. population since the late 1970's can be traced back to: 

 a dramatic increase in coverage of harmful effects of smoking by newspapers in the early 1970's. 

the 1976 ban on tobacco advertising on TV.

the detailed labels on cigarette packages that warned of the many hazards of smoking. 

the wave of health-consciousness that hit the U.S. during the Hippie Movement.