Wikipedia has a cookbook for webmasters looking to increase the ratio of the amount of content they own and the number of search queries they cover [1]. Wikipedia is essentially an army of human taggers that think of ways people might type queries into search engines. They create a page for the alternatives and put #REDIRECT [[Other page]] as the sole text. The MediaWiki software then presents the same content with the alternate title to search engines, and it shows up in your results.

On my local copy of Wikipedia, there are just over 1 redirect per article on average, but the article with the greatest number of redirects is [[List of chess openings]] with 204. I produced a list with the 100 articles that have the most redirects on the English Wikipedia, here [2].

What kind of guidelines should you follow when considering what to redirect? Abbreviations, misspellings, other spellings/punctuation, other capitalizations, other pseudonyms, nicknames and synonyms, scientific names, spelling in other languages, accents, plurals, related words, subtopics or closely related topics and others.

[1] http://en.wikipedia.org/wiki/Wikipedia:Redirect#What_do_we_use_redirects_for.3F
[2] http://pastey.net/30373