While cover letters are very important for your resume, it is equally important to ensure that they are effective cover letters. Much has been said about the contents of a cover letter, but not much in terms of how it has to be presented. It is very important for a cover letter to attract the attention of the employer else the resume will be thrown into the dustbin or worse cold storage.
An effective cover letter is one that reflects your suitability to the job advertised, making you the best candidate for the post. An effective cover letter is one where you state your qualities and achievements in the superlative without exaggerating, to ensnare the attention of the employer. You have to sound as if you are the right person and yet not oversell yourself. A fine line has to be maintained between selling and overselling. For a cover letter to be effective you have to ensure that it is addressed to the right person and the right post. You also have to prompt any future contact at the end of the letter, either by leaving your number or by stating that you will call and find out. This reflects seriousness of the application.
Finally, it is important to read and reread the letter and have it read by others, to ensure that there are no mistakes, spelling or grammatical, as this would defeat the entire purpose.