The U.S. Centers for Disease Control and Prevention created a nine-page guide to Twitter entitled, CDC Micro-blogging Requirements and Best Practices. This is a fantastic document. Not only does it give specific examples on how to use Twitter and how to get those tweets to get retweeted and spread, but it also includes a good history of Twitter and some insight into security concerns that might be involved.
Additionally, if you are planning on using Twitter as an official means of communication from a business or organization, The Center’s Guide will give you a good place to start in creating your own policy and procedure guidelines.