Apple published today a Knowledge Base article that suggests a method to regularly backup your music. This can be done in an incremental way, on either a CD or a DVD using iTunes Smart Playlist features. Basically, you set a rule to show music you added to your library after a given date (the date of your last backup). If it contains more songs than what can fit on a single disk, iTunes will take care of separating them on several disks. When ready, you then press the burn button, enter a CD or a DVD and voilà !
Note the date of the backup on the disk, so next time it’ll be easy to find, or prepare the smart list for the next time, and keep it until you have enough songs to backup.