If you can afford a class, this is the best way to get the tricks and to improve your score. Some places even Guarantee that they will increase your GRE score.

If I were in your position I would take a few practice exams. You can get some in books and the place that gives the GRE test will usually give you free practice exams.
See what your score is if it is just a few points low study the book, if its much lower than you want it to be, sign up for the classes. I have heard from friends that the classes are worth the money, and if they dont help you may even get your money back.

I recommend Grad school, my Grad school experience has been the most rewarding experience of my life. Difficult but fun and life changing.

Good luck and go for it.

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To actually study, what I usually do is this:

read my study notes over a bunch of times. A pass or two to get the overview. Another few to try to get ideas down.
do all of the old assignments over. Refer to the model solutions when checking your work.
now that you know what it is that you don’t know, re-read your study notes and commit as much of the stuff to memory as possible. Flesh out your ideas and understanding. Memorize what you can’t understand.
do old midterms/exams. These are a last check to see what you still don’t know.
work on filling in the last few details that you still don’t know
make a few more passes of the study notes before the exam. This time, try to predict what is in the next paragraph/page.