Overall:
I want to echo what has already been said about doing practice questions and practice tests. It helps you get good timing down and just get a good sense of how you can go about answering the different types of questions that they might throw at you. The GRE is also a test where its a good idea to understand how they do the scoring. For example, I know the GRE is changing but the computer-adaptive one I took had a lot of interesting quirks about how they scored questions. I used the Princeton Review prep book to study and it went over all the important rules of scoring that ended up being very helpful for me.
For the writing section:
The GRE website has sample essays and guidelines of how they do the scoring. I thought the sample essays were really helpful in displaying the type of style and format one should write in to acheive a good score. For me, after I had established what type of format I wanted to use for each essay type, it was a matter of just practicing and getting the timing down.
For the Verbal Section:
I think those lists that Kaplan and Princeton Review have of the most frequently seen words on the GRE are helpful. I studied those. Beyond that, I made a concientious effort to learn new words. In terms of the reading passages, they seemed less intense to me than MCAT verbal reasoning because you'll most likely have more time to devote to each passage. But the types of questions they asked about from the passages seemed very similar to question types on MCAT verbal reasoning. Do a lot of practice questions.
For the Quantitative Section:
Its just a matter of doing sample quesetions and sample tests to get a good sense of how they ask questions and how you can go about solving them. The math is high school level math so I don't think it'll take you forever to review the concepts.
Good luck.
