I have never taught the GMAT, but I was a Kaplan teacher for 3 years and have taught most of the other tests. I don’t know a lot of specifics about the GMAT format, and I know the data sufficiency questions are unique to it, so I can’t advise very well on those.
My general structure for test prep is:
1. Take a practice test BEFORE you do any studying. Use this to diagnose your strengths and weaknesses.
2. Create a study plan that begins by focusing on your weakest areas—but don’t neglect the things you’re already good at. Chances are, even in your strongest areas you’re not getting 100%. With enough practice, you can be.
3. As the test date approaches, revise your strategy to focus MORE on those things you’re already good at. This will nudge your score up and build your confidence.
4. Two weeks or so before the test, start taking practice tests as often as possible to build your endurance and practice your timing.
Beyond that, my general advice is: practice hard and keep a good attitude. People who say you can’t study for standardized tests are people who get shitty scores. Don’t listen to them.
If you want specific advice about question types (especially in the verbal section) I’d be more than happy to help out. Just let me know the question type and I’ll do what I can to advise.