Nice lettuce is a mighty pretty thing, until it shrivels and withers 'twixt store and salad bowl—or, worse, rots and blackens around the edges. Once opened, even relatively shelf-stable bagged lettuces suffer this fate. And lettuce leaves are prone to nasty bruising when roughly handled. This is among the most delicate of foods.
The main storage problem is usually too much moisture. Wet lettuce spoils faster as water condenses on the leaves and suffocates them. More moisture also means more gases, like ethylene, which speed up ripening and spoilage in fruits and vegetables. But here's the rub: Lettuce needs some water to stay crisp—otherwise leaves dry out and droop.
The solution: Keep lettuce moist, but just barely. Loosely wrap a head (or the contents of bagged lettuce) in slightly damp paper towels, and seal in a zip-top bag. This will absorb excess water without dehydrating the leaves. Store in your crisper drawer—the best spot for consistent, controlled humidity. Don't wash lettuce until you're ready to use it.