Make them longer, more random, and less like words. In ssh, a single key and passphrase combination can provide access to lots of other accounts. For these reasons, all of the above rules apply to passphrases even more than to regular and root passwords. Passphrases should be as long as a sentence, but, again, not regular English. Use lots of mangled spelling, wierd punctuation, nonsense words, and mixed case.
Passwords are like underwear.
You don't share them, you don't hang them on your monitor, or under your keyboard, you don't email them, or put them on a web site, and you must change them very often. Many thanks to Kurt Bemis for that quote from his .sig. This worm attacks only Redhat 6.2 and 7.0 machines, gaining access through the known vulnerabilities in wu-ftpd, rpc.statd, and LPRng. If everyone administering a Redhat 6.2 or 7.0 system had applied those updates, Ramen could not have affected a single system. While distribution maintainers do their best to release secure software, new vulnerabilities will show up after the CD is shipped.