Glad you asked...
Keep lettuce in a brown paper bag in the refrigerator and do not pull off the leaves until you want to use them; do not cut lettuce with a knife, pull the leaves off and tear them up to use them; do not wash the lettuce until you are ready to use it.
Then again, this answer might help you...
The best way for keeping lettuce from truing brown is to have proper and good storage for it. wash it and take the leaves apart and place into storage bags.

