My local grocer sells a 1 lb. bag of garden salad for only .99 so I buy it quite often. I was rarely able to use the whole bag before it spoiled until I learned this trick. Once opened, I keep the salad in its original bag and scrunch up 2-3 paper towels. I put them in the bag with the remaining salad.
I then squeeze all the air out of the bag, roll it up and wrap it tightly with a rubber band. This cuts down on the amount of lettuce that browns when exposed to the air. Moisture narrows the short window of time you have to consume it; the paper towel absorbs the moisture and the lettuce lasts much longer.
The next time I dish out my next salad I remove the damp towel and replace it with a new one.