I found a website (through a mention in...the Chicago Tribune? Newsweek? Now I can't remember) called Big Green Purse, which has lots of good information about how we can use our pocketbooks to make an environmental difference.
One thing that really surprised me was whether it was better to ask for paper or plastic when I've forgotten to bring reusable bags. Turns out that the short answer is plastic is better. But there is more to the story than that. (Not sure if this snapshot from the article is readable or not.)
It was part of an article called "Paper vs. Plastic - 13 nagging environmental questions...finally answered." It also had information about whether it's better to turn lights out if you're leaving the room for just a few minutes, cloth vs. disposable diapers, and 10 other topics.
There was a pdf file on the site last summer, but now I can't find it to link to. If you would like a copy (because I saved it to my computer at the time), email me at firstname.lastname@example.org and I'll forward it to you.
I really like these bags. They are about the same dimensions as a paper grocery bag and stand up straight (not floppy like cloth or string bags), very sturdy, and I can put a lot more in one of these than a conventional bag. Ninety-nine cents. The only thing they lack is the ability to say as I'm getting out of the car, "Hey! Take us with you!" I still am not in the habit of grabbing these as I get out.