Fill a clean sink with cool water. Remove and discard any discolored or dried leaves. Detach the fresh, crisp leaves and place in the water. Allow to soak for two or three minutes. Swish around the leaves with your hands to help detach any dirt from the leaves and allow it to sink to the bottom. Repeat this method with a fresh sink of cool water if needed. I eat a piece of my lettuce to make sure it's clean. I don't like the crunch of grit in my teeth while trying to eat a salad. It kills the appetite.
When you declare your lettuce to be clean, shake them over the water and then place them on paper towels. Blot dry with another paper towel or allow them to drain, but don't leave out so long that they wilt.
Another method for cleaning lettuce involves the use of a salad spinner. After separating the lettuce leaves, place them (a few at a time) in the colander and fill the spinner with water. Again, the dirt should sink to the bottom. Lift out the colander to pour out the dirty water. Replace the colander and repeat as necessary until there is no longer any visible dirt. Once the lettuce is clean, put on the lid and begin turning the handle, spinning the lettuce until dry.