Offer to follow up with more information. Don't call a newspaper editor or reporter after 3pm. That's crunch time at most daily newspapers when the staff is rushing to reach deadlines. It's best not to call a TV station just minutes before the newscast or while the cast in on the air. Your message must be of interest to a large part of the audience.
Editors hate it when somebody tries to disguise an ad or commercial as news or an article.
Make sure your written message has good grammar and correctly spelled words. TV programs are looking for something visual. Offer to supply tape showing your event or product. Ezines and tabloids are interested in helpful information you can share with readers. Don't forget radio which needs a constant supply of interesting stories every day of the year.