Here is one tip that I heard anecdotally that might help: it could depend or your time of harvest.
If you harvest during daylight hours, your plants will be fully engaged in photosynthesis and (I'm not sure of the scientific terms) with fully open and active veins and stuff. If you harvest at dusk or night, everything in the plant will be shut off and more solid. My understanding is that big farms usually harvest at dawn for this reason, so the plant doesn't wake up and start photosynthesising before it is harvested - leading to a longer shelf life.
Give it a go! I'd be interested to hear if it makes a difference!
I have also heard what Tim is saying from aquaponic growers. An additional thing I have heard that can be done is dipping the head in cold water.
Drop the heads in some water on the warmer side first then drop in cold water and lettuce will crisp right up ..