Address it to the person who can hire you. Resumes sent to the personnel department have a tougher time of it. If you can find out (through networking and researching) exactly who is making the hiring decision, address the letter to that person. Be sure the name is spelled correctly and the title is correct. A touch of formality is good too: address the person as "Mr.," "Ms.," "Mrs.," "Miss," "Dr.," or "Professor."
Write it in your own words so that it sounds like you--not like something out of a book.
Being "natural" makes many people nervous. And then even more nervous because they are trying to avoid spelling errors and grammatical mistakes.
Show that you know something about the company and the industry. This is where your research comes in. Don't go overboard--just make it clear that you didn't pick this company out of the phone book. You know who they are, what they do and you have chosen them!
Use terms and phrases that are meaningful to the employer. (This is where your industry research and networking come in.) If you are applying for an advertised position, use the requirements in the ad and put them in BOLD type.
Make sure your cover letter contains each of these requirements and shows how you measure up.
