I recommend to start with the FAQ here (viewtopic.php?f=1&t=234
). Many universities have prep sites, UCLA's is pretty good and includes all of the historically released test (http://www.math.ucla.edu/~cmarshak/GREWorkshop.html
I took the GRE a number of years ago. More recently I took the practice tests, and I also took Saturday's exam. In my opinion, the difficulty of the test has increased over time. I have a couple of theories:
1) Universities have demanded more differentiation at the upper end of the scale. It used to be the top score on the GRE was 80%, so there was no way to distinguish the top fifth of candidates who took the test.
2) higher quality, better prepared students are taking the test, especially from outside of the united states. The competition for US grad slots is international and very fierce. So the curve on the test has shifted over time toward more difficulty.
3) More students are familiar with the basic material . So rather than instantly being able to "see" the answer if you know the relevant theorem, the question is written in a more round about way. You have to do more manipulations or make two connections. Since its a timed test, every second counts, even if you know the basic material well. I ended up with a lot more blank questions than when I took the practice tests, and I chalk this up to the length of the questions.
I'm curious what other people think about the difficulty of the test...