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:
Therefore, your best bet is to study the actual words. A reasonable goal is to learn twenty new words every day, while continuing to practice the older words to keep them fresh in your mind. At this rate, you could learn about 600 words in a month.
Since many experts recommend learning about 3,500 words, six months is probably the optimum amount of time to spend building your vocabulary. If you only have a month or two to study, make sure you limit yourself to learning only the 500-1000 most commonly used words. There are many resources, from apps (such as Magoosh's GRE Vocabulary Builder app) to books (such as Essential Words for the GRE from Barron's), that can help you determine which words to learn.
You should also start to look up any unfamiliar words you see; the best way to learn new words is to see them in context. Read publications such as The New York Times, the Wall Street Journal, and The Economist, which often contain GRE-level words. Also, if you see a word you know being used in a way that confuses you, do not hesitate to look it up - words with multiple meanings are very popular on the GRE.
When you are done reviewing your words for the day, most likely with flashcards, a good way to solidify your learning is to practice using the words yourself. Try writing a story or a journal entry using all the words you just used.
For words that you have trouble remembering, using mnemonics can be a good way to make them stick. For instance, the word cacophony (meaning "unpleasant mixture of sounds") sounds kind of like "cough on me," so you could memorize the definition by remembering that having someone cough on you would be unpleasant and noisy. The sillier the mnemonic, the better, because you will be more likely to remember it if it is ridiculous.
Finally, if you must learn a lot of words very quickly, try grouping together words with similar meanings. Instead of learning the exact definitions of 1000 words (for instance), you might only need to memorize the approximate definitions of 100 different groups of words. You may want to invest in the Kaplan Verbal Workbook, which takes a number of popular GRE words and splits them into groups of 10-15 words.
However, if you have time, it is better to learn the exact definitions, as the GRE may ask you to choose between two words with similar meanings but different connotations. Therefore, it is in your best interest to start studying as soon as you decide to take the test.