Press releases are one of the easiet tools to getting the word out about your library. Craft one for large programs, new services and important announcements, but don't inundate the media with them. Take the time to write something that will get attention, and offer something new. You can also send them as followup to a program or event, and include photos or video that they can reprint or post.
If you're planning a big program, or having a well-known author or speaker, you will want to send out a media advisory ahead of time to give news outlets time to plan coverage. This is a simple document that will give the basic information ahead of the event, as well as contact information.