Other than, "good luck," it's hard to know. Remember that whatever you say is going to be interpreted in the context of your decision to leave. So, if they view you as a sharp and knowledgeable person whose ideas should be followed, then perhaps they should follow you right out the door.
The best approach is to try to give them some insight regarding any strategies that helped you get the job done when the responsibilities were mounting. You should also give them the names of other people in the company who were particularly helpful or supportive. Beyond that, ask them if they have any other questions.
If you are so inclined, you can give them a forwarding number or your e-mail address in the event that questions arise later. Don't be surprised if one of these questions is, "Is your company hiring right now?"
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