Five Ws are the most important – Who, What, Where, When and Why.
As Chapa says (hello, fellow hack) a hook is a good thing.
Other good things:
Don't put Press Release in the subject line
Don't attach it as a Word doc or PDF – but the raw text in the email body
Don't start follow up calls with the line: "Did you get my press release?"
If you have images, don't attach them unless requested – put a link in to a selection of images in high resolution. TIFFs are always nice, but good, high res JPEGs will do fine. Don't CMYK them, as different titles have different colour profiles.
If you have the time and inclination, find a hook for each site / mag's readership. i.e, STW and BikeMagic vs. Cycling Plus or the Guardian cycling correspondents – they'll be looking for different things.
Bear in mind deadlines. If you can, speak to someone at each mag and find their news section deadlines, and deliver the release a week or two before it.
Basically, write a short, informative press release, back it up with a few good pictures in good enough resolution for say, MBR to use on a half page, and don't be pushy when talking to journos – they choose what news to report, they're not beholden to you. Give them something that makes their life easier, like a well-structured, relevant release with all the info, and you'll get ink.
HTH