The college admissions season has come and gone. You've turned in your apps, made sure your test score were received and handwritten a thank you letter to Mr. Levy your "bio teach" for his "sweet rec." (This is how I assume you talk.) Nothing to do now, but go back to your normal high school pastimes of attending the ballet and reading Proust.
...you find yourself waitlisted or deferred at one of your top choice schools. If this is the case, 1) I'm sorry to hear that and 2) cheer up! You still have a chance! Most schools will allow you to write a brief update about yourself to them to explain why they'd be missing out if they didn't pluck you off the waitlist. So what to include (and not include)?
The first rule of writing to your waitlist school, is the same as the first rule of life: don't be a jerk. By this I mean, don't mention that you're really upset or angry to have been waitlisted. You can be disappointed, sure, but only to show that this drives you to be even more intent on going and on improving your candidacy. So, stay positive.
In the body of the letter, be sure to include any new improved test scores: SAT, TOEFL, SAT2s, APs (or GPA) are all great because they are quantifiable. You can point and say, "See, I am 15% smarter than when I originally applied. The smart pills I bought online are working." But unquantifiable accomplishments are great too! Just make sure they show the same types of things you were trying to express in your original application: leadership, creativity, initiative. Did you get an internship? Join/start a new club? Take on a leadership position in a club you were already on? Build an orphanage from discarded popsicle sticks? Of course you did. You go, girl.
The last thing to include is why you are so interested in this school. If it's your first choice school, tell them that! They don't want to risk taking you off the waitlist and then having you decide not to go. It also makes them feel good. (This is a love letter after all!) Try to include some information about why the school is a uniquely good match for you as well--and make it stuff you didn't include in the original application if at all possible. Did you visit the school since applying? Cause that would be "tots baller." (Again, I'm just assuming this is your normal way of expressing yourself. You should use standard English in your letter.)
Once you've finished writing this positive and informative letter, address it to the Admissions Officer whose name was on the original waitlist letter and send it off. Then go out and do something fun. Life's too short to sit around waiting for college acceptances!