Start by considering the traditional "five w's" that journalists have always been taught and you won't go far wrong: who, what, when, where and why.
Having established the benefits of our news to the reader and how we might get our press release to work hard in Google, we can get on with the job of writing it, or giving our PR agency the job of doing so. I think the benefit and the key search term should be the first two things on the brief, whether it's a brief to yourself or to someone else.
Start by considering the traditional "five w's" that journalists have always been taught and you won't go far wrong: who, what, when, where and why. Any which are relevant should be in the first paragraph. Don't try to be particularly clever, witty or even entertaining, because journalists will have seen it all before and there's no reason why end-users will either "get it" or share your sense of humour. On the other hand, if your press release is primarily aimed at the UK, don't stick to the type of tedious standard corporate format so beloved of companies the other side of the Atlantic. As experienced technical journalist Dr Charles Clarke told me, anything with "US style boilerplate content" usually goes "straight in the bin". The warning signs on such press releases are introductions like:
"NOWHERESVILLE, NY, DECEMBER 4, 2012: Nowheresville, NY-based The Blue Widget Company, Inc. (NYSE:BYCI), manufacturer of the world-famous BlueWidge(R) Blue Widgets(TM) and a subsidiary of the global industrial components market leader WidgeCorp, Inc (Smalltown, PA), which employs 2,780 people in 16 locations worldwide, announces that…"
…and goes on to bore even the most persevering reader into submission before the first paragraph is out. Why waste your first impression with background information? Another highly experienced technology journalist, Robin Bloor, says: "Being on the receiving end of press releases for around 50 years, for me the golden rule is that the first paragraph should tell the whole story. It is impossible to read in full every press release; I'm sure I would have missed a replacement for the wheel if the release started: "Blogg's Engineering, founded in 1875 under a railway arch in Wolverhampton…" then first mentioned the new development half way down page three. And, believe me, you still get releases exactly like that."
As to length, there are all sorts of views on this, but I've always been of the opinion that you can never provide too much detail. The important thing is that the press release becomes progressively less important as it goes on, so that whenever the reader stops, they've got as many of the main points as possible. If you're uncomfortable with hundreds – or even thousands – of words, thinking that it might put the reader off, then conclude at a convenient point and provide the rest of the information as one or more background articles. Write the piece longer than you'd ideally like, so that you can go back at the end and ruthlessly weed out any unsubstantiated claims which might have slipped in accidentally. Replace these with facts, or get rid of them completely.