One of the biggest keys to writing a cover sheet is to sell yourself to that particular job. A cover sheet may convince the reader that you are a fascinating person, but a company wants a fascinating person who fits the job.
In order to write a specific cover sheet, take time to learn about the company. Use the company’s website or other available information to determine the company’s goals and vision statement. Are they trying to expand? What are their priorities? Has the company shifted emphasis over the past years? If you know what the company wants and needs based on its general philosophy, you can discuss in your cover sheet that you are on board with that philosophy and an asset to their goals.
In addition to understanding a company, however, you also want to understand the specific job opening. Learn as much as you can about the job opening. What are the job responsibilities? What qualifications do most people in the field have? Is there opportunity for advancement? Who will you supervise and report to? Then, again, sell yourself to that particular opening; state why your experience and characteristics equip you to step in and succeed.
When it is at all possible, direct your cover sheet to a specific person. Find the name of the person who listed the job or call the company and ask for the name of the department head. This will give you a contact to follow up with and will make someone in the company take responsibility for your application.
Finally, in some cases, you may send your cover sheet and resume to a company without knowing a specific job opening. This is fine, and you can easily adapt your cover sheet to this purpose. These prospective cover sheets should simply state why you are interested in the company (emphasize that you are on board with the company philosophy) and that you would be interested in discussing any job opportunities.