The emphasis is on calculus and doing calculations. Once you do a practice test, you'll find that there are certain types of problems that are common. So, what I did was find the essential skills that were being tested and practice them over and over. Actually, it didn't work too well in the end because I went for speed, rather than accuracy, but the way it is graded, accuracy is more important. However, I think I had the right idea. You just have to remember to be very careful on the test because you get penalized for wrong answers and the answers that result from the most common inadvertent calculation errors are often available choices.
The two things that I remember practicing a lot were integration by parts and Gaussian elimination. Just do a couple integration by parts problems and a couple Gaussian elimination problems every day, so that you get really proficient at it, even if you know perfectly well how to do those things already. Another common problem is the kind where you have an integral and the limits of the integral are functions, so you have to use the chain rule plus the fundamental theorem of calculus to find the derivative. So, do a thousand of those.
It's helpful to think of integration by parts as "throwing the derivative" from one function to the other and adding a sign change (plus the other boundary term), rather than some big formula that you use that's hard to remember.
The second most important thing is linear algebra. Just focus on calc and linear algebra, even though they seem trivial and easy because the math GRE is about speed and accuracy in calculus and linear algebra, above all else. If you want a great score, you'll have study other things, too, but that's the priority.