I used to own a PR company, and as Gemma said, it's really not rocket science.
Remember the W's...
Who, What, Why, Where, When, and How - okay, how isn't a W but you get the idea...
Make your headline and the first papragraph have a 'hook' to keep them reading. If necessary tailor it to the media you are sending it to...
Here's a reasonable guide.
http://www.foe.co.uk/resource/how_tos/cyw_64_press_release.pdf
If you can (and if appropriate), include a good quality photograph of suitable quality to print.
From my experience the less work a journalist has to do on what you sent the more likely it is to get published. (without being disrespectful - this applies mostly at the lower end of the market, by which I mean local and trade press).
Make sure it's a decent story, fits the media, and that you aren't wasting peoples time.