If you have been waitlisted by your top choice, don't despair! Rather than think of the waitlist as something less than admission, remember that you still have a chance--a chance that can grow significantly with a well-crafted follow-up letter. Unless the school explicitly requests that waitlisted applicants not contact the admissions office, you should write a letter to demonstrate your continued interest in the school, not to mention your determination.
Though this waitlist follow-up letter should adhere to the same stylistic standards as your original admissions essays--concision, specificity, enthusiasm--its focus must be different. Do not simply repackage your personal statement.
Before you begin writing, revisit your application and assess it honestly. What do you perceive as its biggest weakness? If you were the admissions officer reading it, what would make you question this applicant's qualifications? Part of the function of the waitlist follow-up letter will be to address this weakness.
The other major goal of the letter will be to reinforce the assertion that this particular school is ideal for you as you pursue your professional goals. In preparation to write this follow-up, make sure that you can list two or three specific details about the program to which you are applying and convincingly explain why they correspond so well to your career trajectory. If you can't do this, research the school until you can.
Now, it's time to begin writing. Make sure to thank the admissions committee right away for considering your application, and avoid direct references to the fact that you were waitlisted.
Next, address the weaknesses in your application without mentioning them explicitly. For instance, if your grade point average was on the low side, point to good grades you received since you sent in your application. You could also highlight the upward trend in your transcript, expressing confidence that this improvement will continue in graduate school.
This is also your opportunity to mention recent accomplishments that augment your perceived strengths. Perhaps you added another publication or performed more pertinent research, or perhaps you received special commendation for your work. (An additional letter of recommendation or two would complement this letter nicely.) Do not simply restate the achievements already featured on your original application.
Finally, now that you have boosted your qualifications for graduate school, remind your reader why this particular program is ideal for you. Specific details are a must here, and they should be more substantial than simply "School X is on the East Coast, where I could be closer to my family." True as that may be, School X's admissions office wants students who will take advantage of all that it has to offer, not simply its location. Which classes do you want to take? Which professors would you like to know? Essentially, you must answer this question: Why are you taking the time to write this letter instead of simply going to another, perhaps less prestigious school?
Though this may seem like a lot of information, in reality the letter should be about a page long, perhaps a bit longer if you have a number of recent accomplishments to detail. Remember that unwarranted length will not only bore or even annoy a busy admissions officer, but also make you seem desperate. Write succinctly, with all the confidence you display in the classroom, and mail the letter knowing that you have taken advantage of a second chance to sell yourself.