Put the name of a mutual contact in the first sentence.
This is the ideal way to start a cover letter, because it immediately sets you apart from the pile of applicants who have no relationship to the employer.
Tell a story, if you can.
That is a tall order for most of us, but if you can relate your desire for the job to an experience or anecdote in your life, do it.
Near the beginning, briefly summarize your career.
In one or two specific sentences, describe your work experience.
Ilustrate your qualifications with examples.
Give concrete specifics of achievements that illustrate how you could advance the company's agenda.
Conclude by saying when and how you'll get in touch.
In your last line, tell the recipient when to expect to hear from you.
Don't cut and paste from the job description.
If you're applying for a posted job, do keep the requirements in mind, but don't use the exact wording.
Err on the side of formality.
Even though your cover letter will be an e-mail, keep the style businesslike, unless you are writing to a contact with whom you already have a relationship and you are mirroring her informal style.