I work for a Fortune 500 company where I fill about 150 jobs per year so I read A LOT of cover letters...probably over a 1000 each week. The most effective ones are really simple. Instead of creating 4 paragraphs on why the candidate "thinks" they are a match for the job, you should actually take the bullet points off of the posted job description and underneath each bullet, tell the reader how you have already achieved success in that particular area. For example:

Dear Hiring Professional:

I'm submitting my credentials in response to your online job posting on Monster.com for a "Packaging Engineer". I believe that the requirements you have posted directly match my background in the following ways:

- 5 years Packaging Engineering experience
* I have 7 years of Fortune 500 Packaging Engineering experience
- BS in Packaging Engineering
* I achieved a 3.9 GPA at the #1 Packaging School in the country, MSU.
- Experience managing multiple projects through the packaging development cycle.
* I am currently managing 14 full-cycle development projects from concept to commercialization for "Such-and-such Inc.", your competitor.

I would be happy to discuss my qualifications in more detail by phone or in person at your convenience.

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I think you get the picture here. The point is for you to create irrefutable evidence that you can and/or have done exactly what the company is asking for in the ideal candidate. If you are having trouble doing this...chances are you probably aren't qualified and you won't get a response.

For me, cover letters aren't make-or-break but a well-written one that makes my job of matching a candidate to a job easier will get sent to the hiring manager much faster.

Hope this helps!