Leave lettuce heads intact and unwashed until use.
To maintain the proper temperature and moisture level, store unwashed in a loosely closed plastic bag or in the crisper drawer of the refrigerator.
Check lettuce every day or two. Remove any leaves that are beginning to go bad. This will keep the rest of the lettuce from spoiling.
Lettuce bruises easily, so take care not to shove other foods up against it. Using a storage container is better than a bag, which can end up squished against other items.
Make sure lettuce does not end up in the back of the fridge, where it might be prone to freezing.