What to write?
The purpose of the cover letter is to show that you meet or exceed the requirements listed in the job description. It should demonstrate that you’re interested in the position and that you’re available to accept the position if offered. Additional information beyond this isn’t necessary.
What not to write?
Don’t mention negative facts about you. A cover letter is not the place to explain why you left or are leaving an employer, why there are gaps in your employment dates, etc. These “negatives” are best explained in person during the interview so that your personality can counter them. Try to avoid a salary history in the cover letter. If the position specifically asks for your salary history, give a historical salary range, and state that your salary requirements are flexible based on the opportunity the position will provide.
Choose a selling design. Make sure that your cover letter is easy on the eye. Keep in mind, the reviewer of your cover letter has hundreds (if not thousands) of cover letters to look at, so make it easy for him/her to find the information you want to highlight. Keep your cover letter short (no longer than 1 page).
Use a tailor-made cover letter
Avoid the generic cover letter that you get from books. If you are not sure how to write a targeted non-generic cover letter, you should consider using a professional cover letter writing service.