Cut the core out of head lettuce with a plastic knife (even a disposable picnic knife will do). The lettuce won't turn brown as fast if you use plastic. For really crisp lettuce, remove the core and run cold water through the lettuce. Shake as much water out of the lettuce as you can. Wrap in a dishtowel and put in the fridge for a day. Then remove the dishtowel and store the lettuce in a plastic bag. You can put a folded paper towel or cloth in the bottom of the bag to absorb water that may still be in the lettuce.