Admittedly, I was never really sure that cover letters worked as effectively as they do.
I always attempted the uncertain task with trepidation and insecurity with a healthy mix of regurgitation of the main points in my resume.
Recently, however, with some intuitive research and some experience under my belt, I have come to the realization that cover letters are the most integral parts to your job-success and marketing scheme. The trick is how you actually pitch them.
The old approach to writing cover letters is to repeat all information in your resume. STOP IT!
You must express your excitement and enthusiasm for the position you are applying to and also explain, in an example laden way, that you should be invited to an interview.
There's more to it than just that, however, since I now have your attention follow the links and get registered with some employment sites such as www.monster.com or www.workopolis.com if you haven't already done so and inform yourself with the sample cover letters.