Writing a Press Release
SciDevNet
This tutorial covers not only what press releases should contain but also distributing and following up on them.
http://www.scidev.net/archives/hdi_popups/press_release.html

The Definitive Style Guide for E-mail Press Releases
Klariti
E-mail press releases differ from their hard-copy counterparts in several ways. This site offers guidelines for writing successful press releases for e-mail distribution.
http://www.klariti.com/Business%20Writing%20-%20The%20Definitive%20Style%20Guide%20for%20e-mail%20Press%20Releases.html