Alator has the following to say about the GMAT.  Visit the original post for a complete list of advice on getting into a top buisness school.

GMAT: Try and get it out of the way as soon as possible. I had given GMAT more than an year in advance (when I first applied last year). I gave a sample test first to familiarize myself with the structure of the test. I determined what were the areas which I needed to improve and what were the areas which I really did not need to put in much work. In my case for example, I knew I needed to put in more effort in sentence correction while but not the quantitative section. I used The Official Guide for GMAT Review to practice. I tried to do as many problems from the sections where I needed to improve. Finally I gave another full length test to see where I stood. I am really good at standardized tests and did not need to put in more than a month to prepare. You need to assess yourself how much time you will need to prepare for the test.