Include their name on the release, not just the envelope. Only send the release to one person per news outlet . Any problems that develop, from duplicate coverage and effort, will be blamed on you. Don't just send, call. Call the person before sending the release, and call a couple days later to make sure they received it.
Timing is everything.
Don't fax a release out the day before an event and expect it to receive coverage. Give the maximum possible amount of time for the news outlet to decide how they want to cover the story. If you feel the event is so far in the distance that they might forget about it, then simply send another release as the time for the item draws nearer. Know your deadlines. Magazines, even weekly ones, are often planned months in advance.