I've heard that the cover letter is the most important part of your job application, and I've also heard that they immediately go in the circular file. Well, you can't predict what's going to happen to yours, so you might as well make it as good as you can. Scott Reeves at Forbes has some tips on how to make yours the best it can be:

The first paragraph should tell the reader why you're contacting the company and what position you seek. Make it clear that you're applying for a specific job and say why you're interested in the company. In general, it's not necessary to say where you learned of the opening unless that's requested in the ad or you were referred by a mutual friend.

In the second paragraph, tell the reader why you're qualified for the job. This isn't a summary of your attached resume, but a brief recap of its strongest points. Select two or three accomplishments that are relevant to the job opening and back them with specific examples.

It's deadly dull and a killer to cut and paste sections of your resume into the cover letter. Use your experience as a foundation to make a basic point: This is what I can do for you.