Practice, Practice, Practice
Another crucial element of preparing for the GRE is getting lots of practice with the types of questions on the exam. For the verbal section, the biggest hurdle is usually the vocabulary, so you might want to make flashcards and study the roots of certain words. For the quantitative section, make sure you develop systems for doing arithmetic quickly and become familiar with different types of charts and graphs. For the writing section, do practice essays so you'll feel comfortable gathering and expressing your thoughts in a short period of time.
Again, it's important to mimic the conditions of the exam as much as possible when you practice. For instance, don't stress yourself out by trying to do basic calculations in your head because the current version of the GRE allows you to use a calculator. And, because you'll be evaluated for spelling and grammar on the writing section, make sure you don't practice your essays in a program with autocorrect features turned on.