Preparing a good resume cover sheet is as important as the resume or the written application for the post of job you feel suitable for you. A good cover letter is actually going to attract the attention of the employer. Remember that cover letters can be normally of three types- "Application letter" which is a response to the work opening; "Networking letter" that is seeking guidelines for the job search; and the "Prospecting letter" which is asking for the available positions.
To write a good resume cover sheet you must carefully follow the points mentioned here.
Greeting- the letter should have proper salutation to the person who is hiring you. He will probably become your supervisor after you join and he is not the department head. You must know his name and pronunciation; for this you can make the call to the company.
Opening- the resume cover sheet should have a likable opening. Remember that you are basically preparing a sales letter in the guise of the cover letter. State whether you are applying in response to any advertisement or through any person's reference.
Central part- the resume cover sheet's central body should contain the concise portrayal of the qualifications along with reference to the resume.
When you are invited to send your resume, do you do just that? Mail or post your resume and just a resume? What about a covering letter with your resume? Or is that a thing of the past; a mere formality? Well yes and no, a lot of people don't bother with a covering letter along with their resume, but they should. It is not just good etiquette, which it is; but it also makes good business sense. Here is what a covering letter is and how it can help you get your foot inside that door:

It should set out briefly, why in your opinion you are suitable for the job; it is then the job of your resume to give all the details and facts about your qualifications, experience, unique abilities etc. You could say that a covering letter is like a statement and the resume that goes along with it is the corroboration or proof of the statement made in the cover letter.

So assume that one is required even if there is no specific requirement for one. Make it count, by making it arresting and attention grabbing, so that it stands out from among the many resumes that the prospective employer may have received.

The claims that you make in your cover letter though, should be backed up and borne out by the contents of your resume otherwise it would seem as though you are making tall and unsubstantiated claims, which will harm your credibility in the eyes of the prospective employer.

What your cover letter should contain: The cover letter should ideally be addressed to a specific person, so it pays to do some research and find out exactly who in the organization will be dealing with your resume when you send it in. This way you make sure that your resume reaches the correct person and does not go astray. The cover letter should, ordinarily contain your address and date, the employer's address and name of the concerned person, salutation, body and complimentary closing.

So use your resume and its cover letter to make the best impression possible.

Conclusion- the resume cover sheet must conclude with the earnest request for an interview with hint of time and confirmation on your part. Mentioning "sincerely/ honestly/ earnestly yours" as a complimentary closing line.
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