While working in restaurants, I found this is the best way to store lettuce, trust me, it really works.

Take your lettuce, soak entire head in cool water, shake off excess water, remove bruised leaves and cut two inches off the top, place upside down in plastic container and cover loosely with plastic wrap.

This will keep your lettuce fresher longer than if you left it in a bag, plus you take what you need and put back the rest.  It's already clean and will be so crunchy and fresh, you won't believe it.