Washing 
lettuce is one of the easiest things to do and you would be surprised at what lands in the bottom of the bowl! First, chop off the stem and separate the leaves. Place the leaves in a bowl of cool water, leave them soak for a minute or two then shake them off. Place the leaves one at a time on a paper towel. If you are going to use them right away blot dry with more paper towels, if not roll in the paper towels and store in a plastic bag. Lettuce cleaned this way should last about a week.
To wash all fruit and veggies, spray with 3% peroxide, and rinse off after 10 or 15 seconds. The residue will not harm you!!!
Remove the outer bigger leaves and slam the lettuce core down on the counter. This makes removing the core easier. Run cold water where the core was. The water will run all through the lettuce. Turn the core side down to drain. Store it in a plastic bag or lettuce container.
Don’t let the lettuce dry at room temperature, it will go limp. Place lettuce in the refrigerator immediately after washing.
If you don’t have a salad spinner, you can you the hose on your sink to add a little umph to the washing.
I use just a bit of salt in my rinse water. It acts like a mild abrasive.
Don’t forget to wash the prewashed lettuce too! Yuck, who knows where that’s been?
Buy a salad spinner! You can find them at home accessory stores. Basically, they are a bowl with a colander inside and a plunger. The cut up lettuce goes in; you run rinse water over it, lift the colander out, drain the water, then press the plunger to spin the excess water off. Best thing I ever bought!