The cover letter is your chance to demonstrate not only that you are qualified for the job, but that you are eager and willing to perform it. While you will reiterate key points on your resume, a cover letter is not simply a restatement of a resume. You can let your personality and motivation shine without being unprofessional.
Demonstrate that you are familiar with the company, what it does and its mission. Tell the recipient what you do for the company, not what it can do for you or your long-term career plans.
In describing your professional successes, give concrete examples. Don't say that you were a "power seller"; say that you increased revenues by 15 percent. Don't claim that you're a "published author"; tell the reader you've had 24 articles published in industry trade journals. Be wary of cliché phrases. Be specific instead. It's OK to say you're a "self-starter," but elaborate on what that means. Did you single-handedly staff a company's satellite location?
Make sure the skills you list are relevant to the job at hand. You may be great at welding, but that's irrelevant to a desk job. Don't highlight personal attributes unless they are qualities one would look for in an employee.
While you don't want to duplicate all of the information in your resume, you should restate your key qualifications. You can do so in paragraph form, or break out a short list.
Keep it tight. Don't ramble, and don't go over a page in length.
Index of Cover Letter Examples