Hmmm, try a sealed plastic container maybe? I generally just put it in another plastic bag and seal it best i can. I tend to use whats left pretty quickly though, so its not such a worry.
I just store it in the bag it came in, take out what I need, then squeeze out as much air as possible and fold down and clip with a clothes pin (too cheap to buy the bag clips). It lasts a week.
I buy the salad that comes in a resealable bag, but my latest thing is using head lettuce, and cutting it up as I need it. Its cheaper for me, head lettuce is like 1.25 where I can, vs the bag stuff which is over 4.00.