Shave about 1/8 inch off the bottom of the lettuce head stalk with the knife. Take off just enough to remove the old, dry end.
Soak the stalk in a bowl of lukewarm water for about half an hour, being careful not to fully submerge the lettuce head, just the stalk.
Refrigerate the lettuce and after about two hours the leaves will have revived and attained their former crispy state.
Fill a bowl one-quarter full of ice and one-quarter full of water. Mix to make the water icy cold. Leave for about 10 minutes until some of the ice melts into the water. Remove the ice.
Submerge the lettuce leaves in the icy water, leaving them in only for about 30 seconds to a minute.
Remove the lettuce leaves gently from the ice water and place in a plastic bag. Don't shake the leaves, as some of the ice water should remain on them for the best effect.
Place the plastic bag, loosely sealed at the top, into the refrigerator. Wait three hours and the lettuce leave will have revived and be crispy, ready for use.