Here are a couple of bunches of leaf lettuce we had left over from the farmers' market.
First, I cut about 1/8 inch off the end.
Then. I filled the cup about a third full with water and put the lettuce in it.
Finally (and here's the secret to keeping it fresh), I put a plastic produce bag over the lettuce, secured it to the cup with a rubber band and put it in the refrigerator.
I've had lettuce keep for over a week using this method. It does take up some shelf space, but at least it is visible and you are unlikely to forget about it!