Whenever possible, address your letter to a real person. Avoid "To Whom It May Concern." Track down the hiring director, admissions officer or proposal reviewer, who is likely to receive and read your letter.
Match your tone in the letter to the agency or business in which you are applying. A formal tone is appropriate in most cases, but you can be more familiar, if you are applying to an organization that values edginess and creativity.
Be specific in describing what you are applying for in the letter. Refer to particulars of the organization, and explain how it appeals to you.
Convince the reader you deserve to have your application accepted, using specific and concrete examples of your qualifications and accomplishments.
Write confidently. Use active verbs over passive. Choose powerful adjectives.
Keep it focused. Do not write too much; most application letters should only be one page, single-spaced. Choose your strongest examples and edit out anything vague or unnecessary.