My mom taught that a head of lettuce will keep longer if you do the following when you bring it home from the store:
Drop it hard on a counter to break the core. Then tear the core away with a hard twist. Toss the core.
Rinse the leaves under cold water, then dry thoroughly with paper towels.
Don't tear your lettuce into smaller pieces till ready to use. The leaf edges brown first. Less work to tear brown spots off large leaves than small ones.
Store in fridge, in a covered plastic bowl.