Over the long run, extensive reading, but in the short run, memorization is your only option. Can I make a suggestion? I find that when people use flash cards, they tend to go through the whole deck time and time again, and each time they are so overwhelmed by the volume of stuff to memorize that they never learn any more. Next time, start with the whole deck and go through them. Then get rid of all the words that you clearly know, leaving yourself with a smaller deck. Go through those a few times, and a few words will become easy for you. Get rid of those. Basically, you keep eliminating the words you have already learned, leaving yourself free to concentrate only on the ones you aren’t secure with yet. Eventually you will have disposed of the whole pile, and should know all the words.

You’re smart to be looking into preparing–taking the test without preparing is really giving yourself an unfair disadvantage. There are many concepts that you might not have seen since middle school or earlier, and those tests each require a specific mode of thinking. When I was getting ready to take the GMAT, I went with Powerscore on a friend’s recommendation–they have a great weekend class that covers all of the important concepts, with an understandable approach to a test that definitely could have been overwhelming. After class and a little practice, I felt completely confident, because the GMAT is remarkably predictable and I had a strong grasp on the concepts that I needed to understand. I ended up with a 770, and got into my top choice for business school.