I receive hundreds per year, yet have never written one (for a job). I can only offer what does/does not piss me off:

Keep it short, because a long one is a sure sign that it was a form letter sent to every member in the Bar Address book

I would break it into three paragraphs: what you can offer the company, why you are interested in leaving your current job, and what the company can offer you (why you are attracted to the new company). I would close with "I look forward to scheduling an interview with you as soon as possible"

Hand delivery may make an impression--it would at least force me to read it instead of sending back a form rejection letter without having read it.