From the paperkey README file:
  A reasonable way to achieve a long term backup of OpenPGP (GnuPG, PGP,
  etc) keys is to print them out on paper.  Paper and ink have amazingly
  long retention qualities - far longer than the magnetic or optical
  means that are generally used to back up computer data.

WWW: http://www.jabberwocky.com/software/paperkey/