The most important letter in your scientific career?

Hopefully, the title got your attention. This is mostly directed at people applying for postdocs.
The cover letter is key.
I believe most postdoc (and many faculty) applications live or die [i.e., get to the long short list] based on the quality of the cover letter.

You need to specifically answer the following specific questions:

• Why are you interested in this specific job?
• Why are you interested in this specific research?
• Why should they hire specifically you for this specific job?

Most cover letters I receive are generic. People tend to write the same letter for every job they apply for. Furthermore, the research achievements and research interests they list are usually generic.

So to be specific!
Write something like:

"One of the scientific questions I am most interested in is "What is the physical mechanism for XXXX in material YYY? I recently read your nice paper "blah blah" in journal YY and I have been wondering if a similar approach might be relevant to answering my question. I welcome any suggestions from you in this regard....."

Dont write:

"Dear Professor,
I did my Ph.D on topic X and want to continue working on it [even though I have no idea as to whether you have ever worked in this area or have any interest in it]. I published lots of papers [even though most listed on the CV are "in preparation"] and will publish lots more if I come and work with you....]"

The letter is so important you should spend at least a day writing it, even though is should fit on a single page. Should should get a range of faculty to read it and provide feeback.