If using pre-bagged salad, remove the salad from the bag you bought the salad in. Place the bag in a sealable storage bag and put a paper towel in the bag with the salad mix. The paper towel will absorb any moisture so that it doesn't affect the salad. Replace the paper towel every couple of days for the best results.
If using lettuce, clean the lettuce and use a salad spinner to make sure that you get as much moisture off the lettuce as possible. Then place the lettuce in a sealable storage bag and place a paper towel in the bag with the cleaned lettuce. The paper towel will absorb any moisture so that it doesn't affect the lettuce. Replace the paper towel every couple of days for the best results.