i recently sent a letter/pitch to a major magazine. i fretted about it for weeks and then finally just wrote *something*. i let a few people close to me read it and then tweaked the lead.
i haven't heard back from them yet (other than an e-mail saying they received it) so i'm not sure that it was an effective letter but that would be my advice.
write something that touches on your experience, your passion for the subject, your ability to do the job etc.
walk away from it or pass it along to a trusted friend to read.
and then come back like you've never seen it before. if you knew nothing about the person sending it, would you be intrigued enough to want to learn more about them?
keep tweaking. get it down to one well-written page. and then send it off.