The GRE people have a free program on their website. Google GRE and hit "I'm Feeling Lucky." It's on there somewhere and has a couple of practice tests, lots of info about what to expect, explanations of the different types of tests, some practice problems, and lots of interesting statistics about past test-takers.
I just took it maybe a month ago, so if you want any advice or have any specific questions, let me know... I got 800 math, 620 verbal, and 5.5 writing, so I think I figured it out pretty well.
As far as study material... use the online test is all I would do. Honestly, I think either you're ready for a test or you're not. Studying, in my opinion, only is required if you haven't been paying attention in the past. If you know your algebra and geometry the math part is a piece of cake (maybe I'm spoiled since I do a ton every week for class...). The verbal part is more challenging, for sure; you need to read a lot of books and probably take some Latin for basic knowledge of roots, prefixes, and suffixes.
Good luck!