While seeking a job, a cover letter is as important as or more so than the resume. This is because the cover letter largely determines the interest or lack thereof of the employer in your resume. Not everyone is confident of getting that perfect cover letter and they are often rejected by employers, despite their qualifications, because of bad cover letters.
The most common mistake made by many while drafting a cover letter is to replicate the resume. One must most definitely refrain from doing so as this is not going to help. In order to draft the perfect cover letter, it will help to list your achievements and qualification. Once this is identified, the next step is to carefully read the job advertisement and identify the requirements. If the two match then the cover letter should mention where and how the requirements match with your qualifications without repeating your resume.
A very important point that should be covered in a cover letter is the name of the person you are addressing through the cover letter. You can call and find out the name, if it is not mentioned in the advertisement. Also it is important to express your interest in the position and also thank the person at the end for considering you.
Finally before you sign the cover letter proof read it and check for any mistakes, spelling or grammatical and if possible give it to someone in your family or friend circle to read. This would ensure reduction in mistakes in a cover letter.