Nightly automatic backups of everything that's
changed that day (called an incremental backup)
to multiple tapes, with copies of the tapes stored in offsite secure
storage.
If you are just trying to protect your files on your personal
machine, simply making sure that critical files have copies on
multiple physical disks or occasionally copying files onto another
machine or removable storage is probably sufficient. If
you're administering a machine that has multiple
users, regular backups are almost certainly a necessity. If those
users are doing business-critical tasks, very regular backups and
off-site copies are a requirement to protect the investment of time
involved.