According to Jamie O'Boyle, speaking in the article "Plastic Sacked" by Joan Klimkiewicz, "Consumers who shun plastic grocery bags tend to be college-educated urban dwellers who live near stores promoting such awareness, such as Trader Joe's and Whole Foods".
This is precisely why a voluntary and gradual attempt to switch over to cloth bags is just not going to work, however well intentioned. The plastic bags must be banned, not merely discouraged.
It may sound elitist to say, but there exists a vast section of the world's population which will never ever be made to functionally grasp why plastics should be avoided. And even if you do manage to drive this point into their heads, most are too lazy to act upon it. The decision of "paper, plastic, or cloth?" must be taken out of all our hands at the grocery store.