University of Arizona
University of Arizona: Preparing and Distributing Press Releases
An excellent guide from the University of Arizona on writing press releases. Gives detailed information on content and format.
Other
Pr Web: How to Write a Press Release
This site details how to make your press release get noticed. Gives information on content, format, and even gives a template for a standard press release.
Other
Pr web.com: Common Press Release Mistakes
This site is a general list of common errors that are made while writing press releases, both in content and in format.