Thoroughly research the company and determine a list of qualities in an ideal candidate, and do your best to mirror those attributes in your letter. The goal is to convince the hiring manager you would be a valuable asset to the company, even if there are no formal openings.
The first step is to find the name of the hiring manager for the correct department or the contact information for the human resources department, to ensure your letter is viewed by the right individual. Moreover, be sure to articulate the type of position and department you are seeking. Begin your letter of interest with a professional greeting to your point of contact. If you cannot find a name, "To whom it may concern," works just as well.
Your first paragraph should be a strong and captivating statement, highlighting your top two to four assets, that allows you to make a powerful impact on the company. Your second paragraph provides concrete examples of how you successfully exercised those skills in past positions or projects. Your third and final paragraph concludes the letter by expressing interest in setting up an interview or an exploratory meeting, even if there are no current openings. Include your contact information directly below your signature to make it easy for the reader to connect with you.