What's the best thing you can do for cleaner alternatives to driving?
Go to a meeting? Write a blog? Tell your friends? Become a planner? Vote on something?
I don't think so. Although I've done or am doing all of these things, and I recommend them all to you, I think the best thing you can do for clean alternatives to driving is to use them. Walk somewhere, bike somewhere, learn how to use transit (most transit agencies have websites), do something by phone or online instead of making a physical trip, ride in a carpool. Move out of suburbia and into a city neighborhood where these things are easier to do.
Spend your effort supporting the alternatives, giving them a visible presence. Actions speak louder than words. Other people will see you, think about what you're doing and follow you. When enough people act, public investments and urban form even start to change to support the more constructive behavior.