Media Matters has posted these tips for effective LTTEs and making contact with the media (regarding incomplete or slanted coverage):
As a model, please review these reasonable guidelines from Media Matters, for tips on how to effectively communicate with media outlets...
Many media personalities and outlets make their email addresses available online and encourage audience members to provide feedback. Here are a few suggestions to make your feedback as productive as possible:
Be polite. Don't use profanity or personal attacks. Emails that include profanity or insults are rude and ineffective, hurt your credibility, and only offend the recipient.
Keep it short. Limit your overall email to roughly 250 words, or about three paragraphs. You want to make your point clearly and quickly; lengthy emails are unlikely to be read beyond the first paragraph.
Be specific. As much as possible, reference specific news items, articles, columns, and editorials, and be specific about your complaint. Even when presenting a broad complaint (e.g. "Why hasn't your paper done a better job covering the Downing Street Memo?"), it is best to be specific ("Over the last two weeks, your newspaper has not run a single article on the Downing Street Memo.").
State what you want clearly. The recipient of your email should come away with a clear understanding of your request. Do you think an issue has received too little attention? Is factual information presented on equal footing with false information, with the viewer left to decide what to believe? Has an important viewpoint been excluded from a story?
Make sure the subject line of your message conveys what you are writing about. For example, write "Your October 6 article on taxes uses incorrect information" instead of "tax issues."
Personalize your email -- but don't get personal. Each email should be directed to one individual or media outlet and should reference the coverage (or lack thereof) of that particular individual or outlet. Don't write a generic message and send it to a whole list of reporters. Be polite, and keep your email focused on the substance of your complaint, not on personal criticism of the reporter.
Link to further information. Where appropriate, include a link to a website for further information. If you are sending more information in response to a news report, it's always helpful to the recipient of the message to see the source of your information.