I back mine up to cd's by using playlists and sort by date added. I either mark the disks by date or save the playlists to remember the last song backed-up ...

You also have to "corrupt" the iTunes Library.itl file (i.e. create a 0 byte file) so that it rebuilds from the XML file, otherwise it resets all of your editing. It's a pain, and slow. If you're on a ...