I took the GRE last weekend. It's a very easy test I got above 700 on both the verbal and the quantitative sections. I looked through the
Cracking the GRE, 2010 Edition (Graduate School Test Preparation) to refresh on various things and get a brief idea of how to approach the test strategically.
Quantitative
Study for this, you most likely aren't as familiar with middle/high school math as you think you are. Just read through the review book I mentioned above's math section, it has good examples and basic advanced stuff which you are probably not familiar with at all, like combinatorics.
Verbal
If you are a native english speaker you'll get above 500 on this section easily. If not, I guess study?
Writing
Not even acknowledged most places, find out if it is in your case. If so I guess study for it, but for me I didn't need it so I just relaxed and took the first 1:30 of the test without a care in the world.