Lettuces are pretty fragile although romaine is the toughest of them all. For longest storage, wash the heads carefully, even removing each leaf to get out all the dirt (and perhaps a critter or two) that hides between the leaves. Then wrap in something absorbent (I roll up the leaves in dishtowels) and overwrap loosely in plastic wrap. Refrigerate until ready to use. This is one vegetable you don’t want to chop up until right before you’re ready to serve.