The best time to prep lettuce is when you bring it home from the market. But, if you can’t do it then, place in covered container or sealed plastic bag, and refrigerate till a more convenient time.
To begin, purchase only fresh, crisp lettuce. Crisp means that the veins are filled with water. Remove outer, soiled leaves; peel remaining leaves from head, one at a time. Rinse under very cold water till clean. Gently shake excess water from leaves and lay out, without overlapping, on rack or paper towels to dry, turning leaves as needed to dry on both sides. Line a container with paper towels; place leaves in container; cover with more paper towels, then cover tightly with lid and refrigerate till ready to use.
Don’t leave lettuce out too long to dry, otherwise it will become dehydrated and wilt. But, leave out long enough to evaporate water.
To prep romaine and green leaf lettuce: These lettuces require additional prep work to assure crisp leaves. Remove outer soiled and/or wilted leaves. Twist off core about 2-3 inches up from bottom. Discard core and separate leaves. Run each leaf under cold water to clean. Then stand leaves up in a deep, narrow bowl in 1 inch of warm water. Cover with clean, damp cloth towel. Secure with string or elastic band around bowl; pull up towel somewhat to allow enough head space for lettuce as it expands and crisps. Refrigerate overnight in order to allow sufficient time for the water to draw up into the leaves. When leaves are crisped, dry on rack; place container lined with paper towels, and refrigerate till ready to use.
If your lettuce is already crisp when you bring it home from market, wash well, then stand up in colander till dry but not wilted. Then layer between paper towels in large, flat, tightly sealed plastic container.
If that’s too much prep work for your needs, then cut bottom core off plus tips of leaves. Wash, then stand (leaf ends up) in colander till nearly dry. Store in large flat plastic container lined with paper towels. Top with more paper towels then cover tightly and refrigerate till ready to use.
The delight of biting into a perfect leaf of lettuce–plumped with water on the inside and completely dry outside–is worth proper preparation, and will vastly improve the quality of your salads and sandwiches. I know that many people buy bagged, cut, pre-washed lettuce these days. I have never had a bagged lettuce that tasted fresh, smelled fresh or was crisp to the tooth.