Fortunately, it's possible to trick your way into learning a lot of words in a short period of time. (No, it does not involve cheating).
1. Use flashcards to memorize word groups.
Kaplan has a great resource at the end of its GRE Premier 2014 book with a list of words grouped together by similar, overarching meanings. For example, one group might be "timid" and include words such as "recreant" and "timorous," which are both related to the quality of being shy.
I'm not certain if there is a similar online resource for these word groups. I highly suggest getting the Kaplan book anyway and buying a cheap pack of index cards to get these word groups going in your head. Review them everyday, and soon, you'll start to recognize some of the words as you do more and more practice tests.
Use flashcards in general, too. They're quick and easy to use. A tip for students saving money - visit your library's website and see if you can request GRE flashcards either from your college's library or another campus through an interlibrary loan.
2. Look at the origins of a word.
When you're looking up definitions to words you don't know, make it a point to look at the "Origins" section for that word. It's fascinating how words were created; did you know the word "laconic" (which means "succinct") was derived from the name of a Greek region, Laconia/Lacedaemon? Laconians were known for being brief with their words. If you have a story to accompany a word, you'll find that it's easier to remember it later on.
3. Build images in your mind about different words.
I'll give you an example - the other night, my friend came over for dinner and I was looking through my stack of GRE flashcards, trying to memorize the word "opprobrium" (which means "shame" or "disgrace"). I repeatedly said the word "opprobrium" to him while pointing a finger in his face and scowling. He looked confused. I now can remember "opprobrium" because I've connected it to a vivid image and experience.
4. Write it down.
5. Don't use Siri to define your words.
Go ahead. Try it. After 5 minutes, you'll be so frustrated that she can't identify that you said "scurrilous," you will just give up.
These are just a few tips you can try to improve your vocab study sessions. Have you tried anything else and found success (or failure)? Comment below!