Writing too long. They should be no longer than two pages. Sending it too late. Mail or fax it at least two weeks before an event, preferably three or four. Sending a release with no news value.
News is what happens that is different.
If it isn't different, it isn't news. Omitting a contact name and phone number. At the top of the first page in the left corner, let editors know who they can call if they have questions. Calling after you send a release. Don't follow up with a phone call to see if the media got your release, unless you are absolutely sure that someone in the newsroom will check for you.