I send updates a few times per day, at most. I think when people twitter 20 or 30 times per day, that's too much. They are boxing everyone else out, and people stop following them because they need a break.

http://dominiek.com/articles/2008/2/15/how-to-build-a-twitter-agent

http://www.rubyinside.com/building-a-twitter-agent-with-ruby-and-rails-760.html