This is a really great question -- and one that goes into quite a bit of depth -- I'm going to try and give a bulleted overview here.
For a more detailed explanation, you may want to download this free ebook called "The Results-Driven Press Release Formula" here: http://www.onlineprnews.com/how-to-write-a-press-release. - A press release is an announcement of some sort -- it needs to announce something new happening within your company. That might be the launch of an ebook, a sale, a special promotion, or a contest. It cannot be an ad
- Your press release title and summary should give readers an idea of what the press release is announcing
- Your press release should contain keywords that you want your press release to appear for. If you want your press release to appear for "New San Diego Home For Sale," then that phrase should be in the title and summary.
- Your press release should answer the who, what, where, when, why, and how questions that someone might have about the announcement you are making.
- Your press release should include a quote from someone in the company. While most of the press release should be written in more of a objective manner, the quote can be more subjective. This is where you can really let your passion and opinion come through about what is being announced.
- The press release should include links back to your site -- not just your homepage, but any internal pages that relate to the products/services/events mentioned in the press release. Ideally, these links should be placed in keyword rich anchor text and link back to your site (there is more about this in the resource mentioned above).
- The press release should include a boilerplate -- an "about the company" section that is 2-3 sentences that promotes your credentials.
Sometimes it also helps to know what NOT to do when writing a press release.
Here's a link to another free resource titled "The 7 Deadly Mistakes of Press Release Writing" that may be useful: http://www.onlineprnews.com/press-release-mistakes