I went on a week long river rafting trip and learned so many great ideas on storing food. Take the lettuce, wash it, and shake it out. Wrap it in papertowels and store in an air tight gallon ziploc bag. You can reuse the ziploc bags. It lasts for a very long time. I've even used foil when I was staying with a friend and it worked great. I usually store it this way as soon as I get home from the grocery store.
By the way, you can store celery in a piece of foil and it will stay fresh for a very long time. It's the only time I use paper towels, foil, and ziploc bags. I usually use small glass containers to store my food.