I am cheap, and I refused to pay for GRE classes. I did find some websites that I found to be really helpful, though.
First of all.. flashcardmachine.com. If you go there and click on the tab "FlashcardDB" you can search through flashcard sets that other people have made. If you just search for "GRE", you will get a bunch of results. Specifically, if you search for "GRE words - Kaplan" you will find some sets that I personally made using words from Kaplan and Princeton Review guides. I used this site a LOT, and I found that it helped tremendously in learning words.
Also, if you go to scribd.com and search for "GRE," some books come up that might be helpful, especially for learning vocabulary. I remember there being some GRE math and writing books up there, too, but this was last year. I don't know if they were taken down, or maybe they are just harder to find in the search now that a lot more people use the site.
Personally, I was a procrastinator, and I didn't start studying until like 3 weeks before I took the test. Really not the smartest of moves! I spent 95% of my time studying for the vocab section. I learned about 600 words (that I have now forgotten!), but I don't really feel like it helped much. Honestly, maybe 10 of them showed up somewhere on the test total. I wound up getting a 580 on that section, which I'm not very happy with, but I really doubt I can get that much higher. I have always had a hard time with verbal. I can think of large words when I'm writing, and I can understand them when I'm reading, but when I see one just by itself? I cannot tell you what it means.
As far as the math goes.. I didn't really bother studying much for it. I read what the Princeton Review Cracking the GRE book had on it, and I did the practice problems in there. But that was it, really. I wound up getting a 750 on that section. So, go figure!
Everyone has different strengths and weaknesses, you know? You gotta take a practice test or two and try to identify what those are.