Sandra Grauschopf +1 to everyone who said defining the value. What will make it worth a customer's time to engage/visit your community. It is also important to define clearly define how the community supports your organizational goals. If it is driving product adoption, make sure to have a place to share product releases/news, if it is to increase CSAT make sure all posts get a response.
Previously Mentioned Brian Oblinger, has a lot of great resources. Including courses (some free, some for cost), and a podcast (with a resources section) he did with Erica Khul. Richard Millington with FeverBee has a Guide to Creating Community Strategies. Another resources is CMX's Guide to Community Building.
