Refreshments please! 
Board composition these days is expected to be an expertly mixed cocktail of non-executives, made up of a lot of fresh ingredients along with the pickled ones.  Succession planning for the Board is an art as much as a science. Take a good base of relevant skills, combine it with a healthy slug of board diversity and, for an extra twist, add a sitting executive as a NED. 
Investors are increasingly putting the spotlight on succession planning and it needs to be given even more serious consideration than ever.  Each board has a distinctive recipe, but here are a few of the common approaches which can help you find your own special mix and avoid a sour outcome.