I guess it is actually several things. Wi-fi off, don't close the book, don't connect to computer -- here is a discussion on another forum:
http://nooktalk.net/discussion-forum...at-expire.html
I have found that even with the book still open, when wi-fi is on, the auto-syncing (Maybe the automatic clock sync triggers a recheck on expiration dates??) will expire the book whether I have it open already or not.