I studied alone for the GMAT. First I bought the official guide and took the diagnosis test, which was helpful in pointing out how much I would have to (re-) learn. This was pretty scary at first. I then spent about a week revisiting all the basics of secondary level math and English grammar.
After being refreshed on the main key concepts I jumped right into mock tests, which I find crucial for those studying independently. Specifically, I used Manhattan GMAT, purchasing the package with 6 mock tests. These tests were very helpful because they highlighted the areas I needed to improve, so I knew where to focus.
What I also loved about them is that they give you an objective measure of your performance, showing a score which closely corresponds to your actual performance in the real test. I also used flashcards to speed up my mathematical and critical thinking skills.
GMAT score 730 (taken once).
MBA from Harvard Business School, Class of 2013
Founder of Admissions Consulting Firm TopMBA