I have been buying Artisan Lettuce at Costco. It comes in a clam-shell package with six (8, maybe? - can't remember exactly) little heads of lettuce - each a different variety. It keeps much better than the spring mix that Costco also sells. I take the heads out of the package and wrap them in dampened paper towels. Then I place them in a zip-lock bag, but I don't zip it all the way (read that somewhere).