Most recruiters are reviewing a large volume of resumes and your cover letter will likely be scanned and then the reviewer will move on to the resume. Pick one to two areas of your resume to highlight and expand upon in your cover letter.
The goal of cover letters is to expand on your resume by describing the specific skills you have that relate to the open position.
Take the extra time to read the job description in detail and then make your cover letter cover those skills and experiences the job requires and you have a better chance of being noticed.
[S]tate what position you are applying for and why you are interested in the company. Do a little research on the company and reference specific initiatives or products that interest you.
If you do provide a cover letter, make sure that it is tailored to the position - using a generic cover letter does not help your chances.
I believe that 'Dear Hiring Manager' is a good salutation for a cover letter.
[E]nsure that your cover letter really speaks to the job - pull from the job description those items that you feel you will be able to make an immediate and effective impact on and highlight those in your cover letter.
Employers view cover letters as a chance to see if you can write well, and also it provides an opportunity for you to make more direct connections between your experiences and the requirements of the job.
[Y]ou should include your address and the company address on an e-mail cover letter. Even though it is an e-mail, it is still considered official correspondence.