From Martha Stewart:
You've heard this question time and again at the grocery store: Paper or plastic? Well, the correct answer is: neither. Instead, tote your own grocery bag. A single plastic bag can take one thousand years to biodegrade in a landfill, emitting harmful greenhouse gasses. Many groceries sell reusable bags, others offer them free, and some give discounts on your groceries for toting your own bag. There are also companies that donate a portion of your bill to environmentally friendly organizations.