|A mixed bowl of lettuce: romaine and iceberg.|
After trial and error, I've had the most success chopping the lettuce, rinsing it really well in a strainer and letting it drip dry. After about an hour of air drying, I fold several paper towels in half and place them on one side of a Ziplock bag. Place the lettuce inside and squeeze as much air out as possible before sealing shut. Keep in the fridge, replacing the paper towels every few days or when they become damp. For a fraction of the cost of salad mixtures, we have fresh lettuce for several weeks.