Much like writing a resume, the key to writing a good cover letter is to understand how people read them, and by and large, they don't.
People glance at cover letters quickly, first for format and general appearance; this takes from less than a second to a very few seconds. Then they "quick scan" for keywords and qualifications emphasized in the position description and advertisement. Actually a computer may perform the quick scan for keywords before any human even looks at your resume; this is especially true if you are applying online. Because most position advertisements get dozens if not hundreds - and sometimes thousands - of responses, computer scans have become an essential and sometimes preferred way to pre-scan applicants' resumes.
Therefore, crafting a superior cover letter begins by carefully reading and analyzing keywords and phrases used in the advertisement that describe what the employer wants.
1. Most people don't carefully read and analyze the whole ad; they glance through the ad and highlight a few phrases and qualifications that catch their eye - perhaps because those are key skills or attributes that they want to be using in the new position.
2. Many read the ad from their point of view and not from the point of view of either the person who will read their application or the point of view of what the organization has spent the time and money to define, in writing: the person they want to hire.
WHAT TO DO:
1. Go old-school! Print out the position description.
2. Read everything in the ad: the description of the firm or organization, the general description of the position they're seeking to fill, the requirements, the qualifications needed and preferred, and how and where they want you to respond to the ad.
3. Grab a highlighter and highlight everything in all those sections listed above that describes the corporate culture and what they're looking for in the position. Watch especially for words and phrases that are repeated (e.g. they might describe their organization as one that emphasizes "teamwork" and "collaboration," and "building and working in teams" might be listed as requirements or qualifications). The real tone of what they're looking for is usually in the company description of itself ("dynamic," "cutting edge"); if that's how the company sees itself, you need to infuse that same tone and those values in your cover letter. Think: what is the essence of what they want? And what are the keywords or qualifications that they've used to describe the essence?
4. Sit back and analyze what you've highlighted. In addition to identifying the essence of what they're looking for, based on your knowledge of this kind of position, what are the key skills, qualities and personal attributes desired that are also unique to this particular organization? Remember: ads for the same position title are not the same - the emphasis will be different and unique for each company.
5. Craft the opening sentence of your cover letter around the essence of what they want. Incorporate the top skills and qualifications listed in the position advertisement that you have highlighted. Use the same words they used in the ad; those are the exact keywords the reader or computer scanner will be likely looking for. Are there themes restated and reinforced? Use these themes as major headings for copy, then group the bullet points in your letter under them. Match your background and experience to the qualifications and specific requirements of the ideal candidate described. The goal is to have the reader almost instantly get the essence of what you are communicating without having to read the letter carefully.
This is what often makes the difference in those that go in the "yes" pile from those that wind up elsewhere.