Microsoft has released an update that will postpone expiration date of Windows 8 Developer Preview and Windows 8 Server Developer Preview.
Windows 8 Developer Preview was set to expire on March 11, 2012, Windows 8 Server Developer Preview was set to expire April 8, 2012. But after you install this update, the expiration dates for bothe these operating systemw will be postponed to January 15, 2013.
Do note that you must install this update before the expiration date. After the activation license expires, Windows 8 Developer Preview will restart every 2 hours initially and every 1 hour later on.
You can install the update via your Windows Updates or head over to KB2671501 to download it.