Wait-listers - I feel your pain. Last year I applied to 5 schools and got waitlisted at 2 - Darden and USC. I sent both letters, e-mails, made phone calls etc. But what you do will basically depend on the school and its recommended practices. I was fortunate that Darden was open to communication and wanted me to update them when something in my profile had changed.
My major updates included:
A job promotion
Two courses in accounting completed at community college both with A's
One Course in Statistics completed at community college with an A
GMAT score improved from 640 to 710
This was enough to get me in to Darden eventually, my top choice school.
Personally I think if you are on the wait list and you could benefit from some extra coursework - take it. Once I got in I found the accounting and stats were extremely valuable in the first year curriculum and will continue to pay dividends.
If you have a low GMAT score you will need to retake. The promotion and the classes showed the schools I was willing to work but the GMAT score is what ultimately sold them. You HAVE to have the score to get in (my GPA was low too so I especially needed it).
Good luck to all of you - remember this is a test of how bad you want it. Many people said the same things last year: Yield will be high, people wont get off the wait list etc. But if you are willing to committ and put the time in then you will get in. Stay positive and work hard.