Do your research and find out who is the proper person to address your pitch letter to. Once you have found the appropriate producer or reporter and their contact information, make sure you write the letter to them personally. You do not want to send this letter to a generic addressee like "To Whom it May Concern." These generics make it less likely that your letter will ever get into the hands of the proper person.
You do not have to introduce yourself personally in this letter. Instead start off your letter diving right into your pitch. You must grab your reader's attention within the first paragraph or it is unlikely for them to finish reading your pitch. Do not use any moral imperatives like "should" or "must," stay away from these and instead stick to facts.
Next to having the correct address for your letter, facts are the most important component. If you do not have factual reasons as to why the topic you are pitching should be covered, your pitch loses momentum. Including relevant statistics or facts that are from a reliable source will strengthen your pitch by increasing your credibility as an informed citizen who sees an opening in media that should be filled.
Timing is imperative when sending in a pitch. Ideally you want your pitch letter to arrive in the producer or reporter's hands the day a major story breaks that is associated with the topic you are pitching. If this opportunity does present itself, make sure your pitch letter is complete and fax it out to the recipient instead of mailing it. This will insure it gets to its destination at the proper time.