When I was in high school, I felt that the best way to change people was to confront them. Not necessarily in a mean way, but to tell them they were making the wrong decision. I used to be part of the recycling club, and we used to sort and wash all of the cans from the cafeteria once a week. It was actually a really gross and tiring job, since some of those cans had been sitting around for several days (the worst was bottles of chocolate milk). But still, there was a good handful of very dedicated students that, no matter how disgusting it got, were determined to recycle these cans.
There were often garbage cans without recycling bins next to it in the cafeteria, and people would just toss everything in the closest garbage. So every morning, I would drag a recycling bin past rows of tables so it would be next to the garbage. Then when people didn't recycle their cans, even when it was right next to them, I would yell "RECYCLE!!!" at them.
I don't remember how effective this was, but today I can guess that this probably wasn't very effective because generally people don't like being yelled at, much less by a squeaky blonde girl.
Fast forward ten years.
Funny enough, one thing I work on at Sustainable Concordia is to institutionalize the idea that garbage bins can't be by themselves - they need to have recycling and compost bins next to them. But I don't yell at people when they don't recycle. And I don't tell people to turn off their car when it's idling, and I don't freak out when people eat factory farmed meat.
In my personal life at least, I try not to fight the old system. Oh, it's not because the old system doesn't need to be fought - at an institutional or governmental level.
In my own way, I try to create a new system, and support others who are working on the new system. I think the local food movement is a perfect example of this. Simply telling people that it's bad to eat genetically modified corn-based foods grown far away isn't going to do change much.
Give them a super fresh, delicious organic tomato grown on a rooftop or right outside the city, along with an easy recipe so they know what to do with it - now we have the beginning of a change.