To make health and lifestyle improvements that are effective and lasting, I think it is crucial that people develop confidence that 1) success is possible and 2) there is a methodology they can follow that will lead them to that success.
Witnessing my brother's personal health success and his physical transformation through barbell training gave me that confidence. For years, I "knew" I needed to return to exercise and training. No amount of reminding or well-intentioned encouragement would have changed my behavior (or lack thereof). Life always seemed to have had more urgent priorities. Over the years, due to lack of discipline and lack of access to an effective training methodology, I had started different exercise programs with little or no tangible results.
I had not seen my brother for a year or so when I saw his tremendous physical transformation. He was "jacked" as they say! He accomplished this through a compound-lift barbell training program (although it was not Starting Strength). At once, I was convinced that successful health transformation was indeed possible (my brother had done it!) and that there existed a pathway that actually worked that I could follow so that I too could achieve success. I started training immediately, and I have not stopped since. Several months later, I discovered Starting Strength.
Andrew describes many tools we can use to help persuade those for whom we care to adopt healthier lifestyles. I think some of the most effective ways we can do that are by exhibiting our own physical and health achievements (whether that is physical transformation or improvements in performing the physical functional skills of life) and by making people aware that barbell training (specifically, through Starting Strength) is what enabled us to accomplish those achievements.