Your waitlist updates and letters of support from others should focus on three areas:

1. Your qualifications: specifically recent professional achievements, academics, research,  increases in responsibility, initiatives, and community service.
   
2. Steps you have taken to ameliorate weaknesses.
3. Fit with the school.

The first two areas demonstrate that you are an even better applicant today than you were when you applied. The third reveals that you belong at that school like a hand fits in a snug glove on a cold winter day, and that you will attend if, or should I say when, accepted.

Suggestions for a Waitlist Update

1. Briefly thank the school for continuing to consider your application and mention how the school's philosophy and approach fit your educational preferences and goals. Don’t dwell on your disappointment at not being accepted.
2. Agree to take any additional courses or follow any additional instructions provided.
3. Discuss recent achievements. Did you have a 4.0 during the last quarter? Have you led a project or organization? Volunteered? Have you taken your department, business, or club in a new direction? Have you had an article published? Earned a patent? Launched a business? Received a promotion or assumed additional responsibility? Succeeded in a particularly demanding class or project? You should bring out any recent accomplishments not discussed in your application and ideally tie them back to some of the themes or experiences you raised in your essay(s).
4. Discuss how you have addressed shortcomings – without highlighting them. For example, if you enrolled in Toastmasters to improve your communications skills, inform the adcom that you joined Toastmasters two months ago, tell them of any awards you have won, and enlighten them as to how much you are enjoying the experience. BUT don’t say that you are doing all this because you are concerned about your low verbal score or sub-standard grades in social science courses.
5. If you are certain you would attend this school, make it clear that this is your first choice and that you will attend if accepted.

Keep the letter short and sweet -- two pages max. Don’t succumb to the temptation to rewrite or even summarize your life history or essay(s). Stay focused on what you have accomplished since applying.

The above is an excerpt from  The Nine Mistakes You Don't Want to Make on an MBA Waitlist.  If you are waitlisted at medical school or law school, you can find the advice you need The Nine Mistakes You Don't Want to Make on a Medical School Waitlist or The Nine Mistakes You Don't Want to Make on a Law School Waitlist.