I have fresh lettuce from the garden, and if you pick it and just stick it in the fridge, it'll get wilty. So what I've been doing is wash it, then dry it a bit and wrap it in a dish towel, then put the whole thing in a plastic grocery bag. It stays crisp for quite a while.
However, I've had a wilty piece that I put in Ralph's tank, and since he only goes to lettuce as a last resort, it was untouched for a couple hours, and by that time, it had crisped up again