This is the way I prepare and store lettuce--CSA, my own garden, or grocery store. It's reliable and doesn't take up much space in the fridge; even better: the lettuce is ready to go when I need it. This will keep your lettuce in good shape for a week or more.
If the lettuce is very dirty, let the leaves float for a bit. The dirt will sink to the bottom and you can scoop the leaves off the top. Notice all the dirt left after a few heads of farm fresh lettuce! There's also a little friend I found lurking on one of the leaves.... :-)
Spin! (Alternately, you can put the leaves in a clean dishtowel, grab the 4 corners up together, and spin the ball around--outside.)
Place in a gallon-sized ziploc bag, seal, and--very important--slash a few holes in the bag with a sharp knife. I like to reuse my bags, so I label them "lettuce" so I won't reuse a hole-filled bag for something else later on. Store in fridge until ready to use. You should get a week or more out of these bags.
Do YOU have a favorite lettuce prep or storage tip?