First of all, let's get straight what a press release is and is not. A press release is a short article written in a factual way. It is not a sales letter or an advertisement. Its main purpose is to promote your product, company, or event to the media in order to gain publicity. A well written press release can make a reporter's job easier by providing the basics for a news story, but a poorly written one will be discarded.
The most important thing to remember about a press release is that it must be "newsworthy". Newsworthy basically means something that is interesting enough to be, well, news! The five main criteria used to determine whether an item is newsworthy are its timeliness (is it current?), significance, location (how local is it?), prominence, and human interest value. For example, a men's clothing store giving Tie-tying tips to dads for Father's day is news (timeliness, human interest), but having a Father's Day sale is not. Remember, it has to be interesting to those in the media and to your audience, not just to you.