I took the GMAT on March 9 and scored a 680. However, I need a much higher score to even have a chance at some of the top business schools in the 3rd round. I plan on retaking the exam on March 31, which gives me 5 days of studying. Is there any advice anybody can give me to tell me what I should do to study in these next 5 days?

How is that even possible? I thought you had to wait 31 days between GMAT retakes. Were you able to register for a Mar 31st timeslot through mba.com?

What are your weak areas? You probably just want to pick one or two areas and do a couple days of concentrated studying. In general it's probably hard to gain much in less than a week. Were you scoring a lot higher than 680 in GMAT Prep? If that's the case perhaps you just need a bit of luck this time around.

Good luck.

This might be too late for you, but just in case (and for future reference for other readers):

The GMAT is most definitely not a test for which you can cram. Most of the skills needed on the GMAT - logical reasoning, problem solving, mental flexibility, etc. - are subtle and require long periods of practice to attain. With only a few days, you should focus on the two things that you really can cram for: grammatical rules and math formulas. Even that, however, can only take you so far; the real issue in sentence correction is not the grammatical rules themselves (they're pretty simple rules for the most part), but spotting them in sentences. Similarly, the the formulas required in quant are usually pretty basic. It's having to solve a problem that's worded oddly or that has traps built into it that's the tough part.

(and yes, you have to wait 30 days between GMAT attempts)

Hope this helps!