I don't know about the bags of mixed greans, sometimes they last quite awhile and other times it gets nasty pretty quick. With ice burg and romaine, I've found that if I wash it and leave it in pretty big pieces and then dry it really well, like with a salad spinner and then several sheets of really absorbant paper towels, place it into a ziplock type bag with another clean, dry paper towel it lasts for awhile longer. I think the thing is to keep it dry.
The red and green leaf lettuces seem to have more water content or something, I've not had good luck with them at all. I have to use them with in a day or two
I suppose it would work with the bags of mixed salad but I'm too busy (lazy

) to try it. I check the expiration dates on the bags and try to find the one with the longest.
Hopefully others will give you ideas also.
Sarah