Some Points to Consider
Put your name and contact info at the top of the letter. Make sure your name is in a font large enough to easily pick out.
Whenever possible, address the letter to a specific person by name and title. Even if responding to a job ad that states "no phone calls", consider calling to politely ask the name of the hiring authority. Unfortunately, you may not always be able to identify the name of a specific person. In which case, send the letter to the title of the recipient (i.e. Human Resources Manager, Recruiter, Marketing Manager, Accounts Payable Supervisor, Office Manager, etc). Do not use "To whom it may concern."
State your interest in the job for which you are applying and the company. Be particular about why you really like this position and company and not some other position and company.
Mention your skills, education, special training, and work experience that qualify you for the job. Try to think from the employer's point of view - what skills and experience are they looking for in the ideal candidate for the job you want? (If you are replying to a specific job ad, this may be listed there) Stress these skills.
Provide a contact info (phone number, email) that you can be contacted easily or else somewhere where they can leave a message that check regularly (if need be give the best time the employer can reach you at).
Use an appropriate closing, such as "Sincerely."
Ask someone to proofread your letter to check content, grammar, and spelling.
You may want to be proactive and state a general time when you will call the employer to follow-up.
Try to sign the letter in blue ink since it implies the letter is original and may get more attention. Also appropriate ink color to use is black. Never use any other color on the cover letter.
Keep sentences short and to the point....