Vocabulary is one of the biggest and most important aspects of the GRE. You will need to start studying vocabulary well before you take the test, because it is a slow process. There is a limit to how many words you can effectively study at one time; it is impossible to learn thousands of words by cramming at the last minute.
There are no real shortcuts, either. Some people try to learn root words in an attempt to minimize study time, but this technique does not work very well on the GRE, for various reasons: