I store the washed and dried lettuce in a gallon size Ziploc bag lined with paper towels. Seal the bag and store in the fridge just like you would bagged salad from the store. The two heads yield about 13 oz. of chopped lettuce, which makes six side salads, or enough to last my small family for three meals. I chop the other heads as we need it and when I have time.
Making my own pre-bagged salad works for me. For other tips, check out We Are That Family.