I took the GMAT two years ago. I didn't take any prep courses. I went to Barnes & Noble and bought a few prep books (and used the two that Glo is sending. Thanks again!) You just have to be really dedicated to studying. I may have overdid it, but I studied 6-8 hours a week in the two months prior to taking the test.

My advice is to go through a prep book or two to refresh everything, then skip the stuff you know well and just focus on your weak spots. I pretty much only studied math for the last month.

Also, there are two practice tests that you can download from the GMAT site. They're really helpful. It's set up like the real test, with timers and everything, so it helps you get the feel for everything.