Old chef trick, this!!!...
Run a sink of icy cold water and empty the leaves into it. This will refresh them.
Take them out and dry as best you can without bruising the leaves (a salad spinner is ideal).
Take some kitchen paper (or ideally, the thicker kitchen tissue if you can) and plunge it in the water. Wring out so it's only just damp, and place a piece in the bottom of a bowl or whatever you're using to keep your leaves in. Place your leaves in, then put another sheet of dampened tissue on top. You'll be amazed at how fresh it keeps the salad!