This has really worked for me.
I don't buy the bagged organic lettuce and spring mix unless I will use it that day (gets soggy and can't breath in the bags).
I buy a head of organic lettuce (red and/or green leaf is good, the darker, the better). No wilted leaves on it.
When I get home, I take off the plastic bag (throw away) and tear off about 4 sheets of papertowels, all still connected to make 1 long one. I wet the papertowels and lightly squeeze out the water.
Then I wrap the head of lettuce loosely with the papertowels and put it in the bottom drawer of the refrigerator.
It can last about 10 days.