The hallmarks of a good prep course is one that teaches all of the content that you will see on the GMAT and does not focus on content that is not tested. This will make your study time more efficient. Secondarily, you need a prep course that focuses on strategy and timing as well as content. Once you begin studying you will realize that the math tested on the GMAT is basic up to Algebra II but the reasoning is much more difficult. You need someone who can help you with all sides of this and who also has plenty of practice exams that you can take to gauge your progress.
I am an online teacher/tutor for The Princeton Review and have excellent results with my online classes. We will be launching a new class for the GMAT begining April 30th - this new class will include 26 hours of in class lessons that are recorded for later viewing, 3 additional workshops to target problem areas, 10 practice exams and 3 full books of content along with the official guide. With our previous course I have helped many students succeed and I believe the new course will provide any student with the grounding they need.
Feel free to contact me for any additional information.
Best of Luck
Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA