We really really really encourage you to buy food and eat it right away. However, if you must get a head of lettuce and stretch it out a week, we encourage use of Biobags. These bags allow the food to breathe, keeping food fresh for longer periods. And, since they're made out of corn, they're biodegradable. Can't say the same for plastic, now can you?
We first saw these at our neighborhood natural foods store, Lifethyme, and have since seen them at Whole Foods as well. Treehugger did a little review of the bags a while back, which is where we learned you can now order the company's full line of bags (food scrap, trash, pet waste...) online at Drugstore.com.
So now when you forget about that head of lettuce for longer than even the Biobag can preserve it, you can just toss the whole thing into your compost (or second choice, trash bin) guilt-free.