If your cover letter does not sell you effectively then your resume may never be read. It should be typed on the same paper on which your resume is printed, and both should be sent in a matching envelope. Either traditional business format or blocked format is acceptable, but be consistent throughout. Although you may develop one basic letter, you must avoid sending what seems like a form letter. Type and personalize each letter individually for each company.
Employers recognize form letters very quickly.
A well-written cover letter can make a critical difference in an employer's estimate of your qualifications. An applicant with average credentials may gain many interviews with an excellent cover letter, while a well-qualified candidate may defeat him or herself with a poorly written one. Especially in fields where writing skills are essential, you must have a superior letter. The letter should give convincing reasons why you are interested in the industry, and why this company appeals to you. You should leave no doubt in the employer's mind that you know something about his or her company.