I take it you are talking about the general test and not the subject tests. Here’s what worked for me:
First, I took a practice test. If you are near a university still, there might be free sessions from time to time that simulate the exam. If not, I believe you can download practice tests from the ETS website. These are pencil and paper rather than computerized, so it is not the same thing. But you’ll get some idea of what to expect and what the grim reality of your current position is (at least, it was grim in my case).
Second, I bought the most recent Barron’s book on the GRE and came as close as I could to memorizing it. That is to say, I studied it as much as I could for an entire month. A lot of the questions involve “tricks,” especially the math questions. They are testing concepts and how quickly you can see that you are dealing with a certain kind of problem.
That said, I wish I could have afforded a Kaplan class. They have a lot invested in making sure you do well, and they routinely turn out successful test-takers. As an educator, I’m actually a little suspicious about the relationship between Kaplan and ETS. The GRE seems to be designed in such a way that the more expensive of study materials you can afford, the better you’ll do. But despite the exam’s limited predictive value, people still set quite a bit of store by GRE results. There’s really no choice but to play along. And a Kaplan class is definitely how they prefer you to play.