Blue Avocado: Twelve Ways to Get a New Executive Director Off on the Right Foot
This great article from Tim Wolfred and Jan Masaoka has some very basic but effective things that the board can do to help get a new ED started out the right way. It's a quick read and worth your time if your nonprofit organization is in the midst of or expecting an executive transition.
Even if you're not in expecting a transition, you can decide and define how a transition process will function, from who will be notified when your ED does decide to leave to decisions like those in the article on how to onboard your next leader.
The best time to make these decisions is before you need to use them.