I have had Romaine lettuce and curly lettuce stay fresh for weeks by making sure that air can get into the plastic bag. I just close the bag with a single clothes pin. If it is airtight, the lettuce gets slimy. I do turn the lettuce so the leaves are at the bottom of the bag and the bottoms are near the opening. Some people suggest putting a folded paper towel in the bag.
May you live as long as you wish and love as long as you live.
Robert A. Heinlein