If you want to appear professional, you must use the AP Stylebook. Probably the biggest reason behind press releases not being picked up is that they aren't news. No matter how excellent your journalistic writing style, a press release will go nowhere if it is not newsworthy. While that subject can open the door to considerable debate, here are some guidelines. News is that which interrupts the status quo.
News is marked by several attributes that can increase or decrease newsworthiness.
These attributes are timeliness, proximity, prominence, unusualness and consequence. Study those words carefully. Commit them to memory. You can use them to test the news value, the importance, of your story to the news media. When writing your story, think about what is important to the audience.