Thoroughly read and understand all the instructions for each section/test ahead of time to save you precious time when taking the test.
Start prepping early, to give you time to create your own study guides and answer lots and lots of sample questions.
Take as many sample tests as possible! Not only will this train you to answer the kinds of questions on the test, but it will greatly reduce the stress factor when doing the real thing.
Create your own vocabulary cue cards from past and sample tests. Learn 1 or 2 words every day to build your vocabulary.
Unlike the paper version of the test, the computer version does not allow you to skip questions and go back to them later. YOU MUST ANSWER QUESTIONS IN THE ORDER THEY ARE PRESENTED.
The test will start with easier questions, so don't spend too much time on the first few. HOWEVER, the level of difficulty for the following questions is based on how you answer each preceding question. Higher level of difficulty = higher score.
If you don't know the answer to a question, try to eliminate as many wrong answers as possible. For every incorrect answer you can safely eliminate, your chances of guessing the correct answer go waaay up!
In the analogy section, create sentences that show the relationship between the two words. Try to make the relationship as specific as possible, even if you must reverse the terms (just make sure to reverse the terms in the answer set too!)
For example, in the analogy page::book, the sentence "A book is made up of many pages." is better than "There are pages in a book."*
Know your special right triangles! In the Geometry portion of the GRE, questions often use certain kinds of triangles whose sides and angles follow special rules. If you know the rules ahead of time, you can save yourself a ton of time on calculations.