You may wonder if washing lettuce is actually a "technique," and if so, whether it’s worth describing. I met my wife when she found a caterpillar--half a caterpillar, actually -- in her salad at my restaurant, and she has never let me live it down. So let me tell you how I [now] clean lettuce.
Start with the freshest, best-tasting lettuce you can find. It can be a single variety like watercress or arugula or a mix of leaf lettuces (sometimes called "mesclun"), a mature head or baby lettuces, whatever looks crisp and garden-fresh.
Fill a large bowl or sink with cold water, making certain that whatever you container you choose will hold a ratio of 3 parts cold water to 1 part lettuce. When the container is full, gently submerge the lettuce in the water. (I don't like to run the water over the lettuce as it can bruise the delicate leaves.) There should be plenty of room to gently agitate and move the lettuce around, causing any dirt or debris on the leaves to come loose and fall to the bottom of the container. The process of washing may need to be repeated more than once, especially with winter lettuces that have been subjected to rainy weather and muddy soil.
When the water is almost completely clean, use your hands to remove the lettuce leaves from the water and place them in a colander or salad spinner. You don’t want to dump the lettuce and water out at the same time as you’ll reintroduce the dirt and debris you’ve separated out back to the lettuce leaves. Dry the lettuce in a salad spinner, removing the water at the bottom of the spinner between spins if necessary, and chill the lettuce for 30 minutes before assembling your salad. This will bring crunch and crispness to the leaves.
When you are ready to serve the salad, toss the lettuce with a modest amount of vinaigrette. You can always add more, but you can't take it away if you add too much dressing to start. If you are incorporating other vegetables or fruit with the lettuce, toss those ingredients in the vinaigrette too. Cheeses and crunchy items like nuts and croutons should be added to the top of the dressed salad.