Your cover letter should be completely customized to suit your job application, so don't just have a generic one and change the top paragraph to include the name of the company. The generic introduction is found in your resume; your cover letter should be the frosting on the cake, as it were. You can, of course, write a generic cover letter (make sure it's a great one) and then customize it to suit each particular job application. This will save you a great deal of time.
The heading should include all your contact information. Make sure it's up to date, and if you move, send a new resume and cover letter with your new contact details. You will need to draw your prospective employers' attention to how well your credentials meet the need stated in the advertisement you are answering. You will also need to demonstrate that what is important to the employer is also important to you.
The first paragraph should contain information about the specific job you are applying for. Don't just assume there is only one vacancy. If there are several, it will cause confusion, making it very easy for the reader to toss your letter into the wastebasket. Include information about how you found the job opening and your reason for sending your resume. This may seem obvious, but it is also polite and will make things very clear from the outset. Mention the company's products, philosophy and reputation to show your interest. As you are sincere, employers will appreciate your interest. If you know someone who works for them already, it's okay to name-drop, but be sure that person will give you a sincere recommendation.
The second paragraph is where you should name the benefits that you can offer your employer. You need to be able to demonstrate that you can offer specific benefits and value to an employer. If possible, give specific reasons why you are the ideal candidate for the job. If you have great organizational skills say so, and show how you can apply this to the job.
The closing paragraph is important too. This is where you let the employer know you will follow up to confirm the receipt of your resume and make an appointment for an interview. If you think this seems too bold, remember that you should always assume a positive outcome and thank the employer in advance for the interview. This may be all it takes to swing things in your favor.
Visit http://www.greatjobguru.com and learn about the many other steps to getting a great job or promotion. Your cover letter helped you get in the door. Know let the GreatJobGuru help you complete the final steps in securing a great future.