My lettuces don't get the chance to develop whole "heads" because I keep picking the outside leaves -- just enough for the meal.
Store-bought lettuce heads, I wash by swishing the whole head in a big pot of water. After draining as much water as I can, I re-cut the bottom and press against the cut a wet folded sheet of paper towel that's been wrung out just enough so it's not dripping, then put the head in a giant plastic bag (loose enough not to compress the leaves) cut-end first, taking care that the paper towel stays in contact with the bottom of the lettuce. Top of the bag is loosely folded over. If the bag is not big enough, I loosely over-bag with a 2nd bag, or put a moist tea towel over the leaves (but you have to keep it moist). The lettuce goes in the fridge where it's not going to get frost damaged (not very top or very bottom) standing up.
Hope that helps