As a Kellogg MBA, the single most important piece of advice I can give is to register for the test. Pick a date and pay the money. You will never be as prepared as you like. You can study indefinitely. Having the date will help you stay on track and focus on the areas you really need to work on.

The second best piece of advice I can offer is to take as many practice tests as you can. As you take each test, identify questions you've guessed on. When you're done with the test, go through every question you got wrong and every question you guessed on and take the time to understand that problem. This will take you a long way.