Every 1 to 2 months, I clean up all my files and put them in various directories in my home directory, I tar them all up, tar up my dotfiles, tar /var, tar /etc, and if needed /usr/local.  I then burn all of these to cd.  A few times a year I do a big mp3 backup, but that's alot of cd's, while a normal backup fits on 1 cd.