Okay, you're going grocery shopping and you've forgotten that all-important reusable shopping bag at home. (You really should carry at least one extra bag in your Prius. You do drive a Prius, right?) So when the cashier asks you for paper or plastic, you automatically choose paper, because it's better for the environment. Wrong choice. Though plastic takes, like, 17 quadrillion years to biodegrade (alright, I exaggerate), here's why it's slightly less of an awful alternative: producing a plastic bag requires 20 percent of the energy that producing a paper one does; paper bag production creates 50 times more water pollutants; and it takes 91 percent less energy to recycle a plastic bag than a paper one. You can find all of the info here.
Practice what I preach? I practically bust the hypocrisy meter on this one. I now get plastic bags instead of paper when I forget to bring my reusable cloth shopping bag. But truth be told, I've never recycled a plastic bag. (In my town, I can't throw it into the recycling bin I keep by my trash can. I need to take it back to the grocery store, and stuff it into a special "plastic bag recycling bin" there.)