Being on the waitlist is a tough spot. Many students each year get on the waitlist and cannot do much other than wait. For some on the waitlist, schools will accept new GMAT scores - if there is substantial improvement then this can be enough to bring that candidate off the waitlist into admission.
However, this option is not a guarantee. We've had students waitlisted who then retook the GMAT and went from 690 to 760 - they were immediately admitted in. We had other students who were waitlisted at 690 - she thought about retaking but never scheduled it. And miraculously she got off the waitlist and was admitted without doing anything!
So, you really have to look at your background and your odds and see if you come in as unique an applicant as possible to increase your chances. After all those factors, the only remaining component that can make you stand out is a great GMAT score.