You can't exactly. You can back it up: If you back up both the System State data and the folder in which the Terminal Server License Server is installed, then data in both the registry and the Terminal Services licensing server database is backed up, and if you restore this data and database to the original licensing server, then the unissued licenses should still be available. However, a new server will cause the unissued licenses not to be restored. You'll need to call the Microsoft Clearinghouse to get the licenses back.
This was first published in August 2003