The best method is to backup the library to an external hard drive frequently with a backup application that can do incremental backups like
Synk Standard. After the first backup it only takes about a minute or less to make the incremental backups.
Next would be to do as Niel suggested. If the library is too large for one DVD disk, rather than burning the library parts piecemeal burn groups of Events (about 4GB worth) to disk using the Share->Burn menu option in iPhoto. This creates a mini library containing the original and modified versions of the photos in the Events you've selected along with the keywords, titles, places, etc. The photos can be copied back into a library by mounting the disk while iPhoto is running. It will show up in iPhoto on the left.
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There you can select the photo, event or album you want and drag to the Events icon above to import.
As you add more Events as time passes all you need to do is select the newest Events and burn those.
OT