Read the mission statement of the college or university to which you are applying. Make sure that this college or university is a good fit for you--writing a cover letter that is appropriate and interesting to the admission board will be much easier if your interests and the college or university's mission statement align. Write down anything that impresses you about the mission statement.
Select and write down your three most impressive accomplishments, and your academic background with any honors you may have received. Link your accomplishments with the mission statement of the school to which you are applying. Make the connection between you the prospective school explicit--this will guide your writing and it will make your cover letter stand out. It will also convey your interest in the school to the admission board.
Type your address and the date on the upper left hand corner of the page. Type the school address below this. Open your letter with a professional salutation. Write your academic background and achievements in the opening paragraph. Include your accomplishments and specific strengths in the second paragraph--here is where you make an explicit link between yourself and the school. Write briefly why you wish to attend this university or college in the final paragraph. Close the letter by tying together what you have already written. Reiterate any points you feel are especially salient.
Write clearly and simply--the cover letter should display your writing prowess through your ability to present the necessary information clearly concisely. Make sure your cover letter is no longer than three paragraphs--it should easily fit on one page. Proofread--this final bit of effort could be the difference between a discarded application and an opportunity to land an interview.