The general rule when buying lettuce or other greens is to look for those that are crisp and free of blemishes. They should smell fresh, never sour. Greens will last longer if they're washed as soon as you get them home.
The easiest way to clean greens is to put them in a sink or large container full of cold water. Swish the greens around with your hands, then let them stand for a few minutes for the dirt to sink to the bottom.
All greens must be thoroughly dried before using or storing. I use a salad spinner (the best $15 investment I ever made). Rather than overload the spinner, do the greens in batches. They can also be blotted dry with paper towels or a cotton dish towel.
Wrap clean greens loosely in paper towels, then put them in a tightly sealed plastic bag. Remove as much air as possible from the bag before sealing it. Greens prepared and stored in this manner can be refrigerated for up to a week.
Source: Cooking Smart by Sharon Tyler Herbst