Whenever I talk about environmental issues, I always try to avoid being preachy or freaking people out. Yelling at someone for not using their own bags won't solve anything; it will most likely make them retaliate - in a non-violent, yet, non-green way.
On WNPR's "Where We Live," there was an interesting segment about "green guilt" and how the fanatics who religiously ascribe to the green movement can turn people away. Persuasion is one thing, but force and pessimism is another. Take a listen (Colin Beavan of "No Impact Man" is featured at the beginning!): WWL: Getting to Green | Connecticut Public Broadcasting Network
We should approach any change with positivity and an eagerness to educate others in a less scary, yet convincing, and perhaps life-affirming way.