The GRE test if probably is the most important Graduate exams one will write. This exam is required for all Universities in the US and quite a few top Universities in Europe also ask for them for example like ETH Zurich. This exam has become more important off late because a lot of management graduate schools have also started accepting GRE scores as an alternative to GMAT scores.
That being said, I am an engineer who graduated last year to pursue my Masters in Sustainable Energy Technology right after my Bachelors. I took my GRE in 2012 after the new pattern was implemented.

GRE Attempt – December 2012 – 308 (Q: 159; V: 149; AW: 3.5)

Personally, I believe, this is a really bad score if you want to get into a really good university in the US. I believe that a minimum of 320 should be every GRE test takers target.
My bad scores have little to do with the material I used more to do with my preparation. And the reason I didn’t bother retaking it was because I was concentrating on Universities in Europe more than the US.
The Resources:

1. Barrons Book and CD (For Math and Verbal)

The book is decent and so is the CD with the 2 exams. But, it is not enough. No one book is enough to prepare for the GRE. It is always advisable to use multiple books for more practice and to get acquainted to different styles of approaching problems, this is especially very important when it comes to the verbal section.
While I have heard pros and cons for almost all the books available in the market, let it be the Princeton Review, Kaplan, the Official Guide, the Manhattan Books, but let me assure you that do achieve an undeniably awesome score the more one practices using different sources the better it is. All the reviews are based on personal opinion and hence I conclude that there is no extremely bad book and there is no extremely good book. Every book in the market is just the same. The Kaplan, Princeton and Barrons are more or less the same. The Manhattan books are better for more practice and the OG is also good for the same. I would suggest try to do at least three of the above books {my suggestion on the 3 books: Kaplan, Princeton, the OG (the OG has to be one of the 3 books)}

2. Word Power Made Easy (For Vocabulary)

This is probably the best book out there to properly memorize the plethora of words that is expected to by-hearted for the GRE. Be patient with it. Give it time and for heaven’s sake follow every letter of the instructions as mentioned (I messed up there). Give the book at least a good 2.5 or 3 months to do properly. Then, finally, concentrate on the list of words mentioned behind like a lot. I found a lot of words from that list presenting itself during the actual test. The reason this book is probably (at least in my view) the holy grail when it comes to properly learning English vocabulary is because it not only teaches you the meaning of each of the words but also explains the word it’s root and the context in which the word should be used (which is extremely important in GRE). But as I mentioned before this is not enough, the other sources which I would strongly recommend are:
a. The Barrons 333 High Frequency list
b. The Kaplan High frequency word list
c. The Magoosh Vocab E-book. They a book which has only words and one which has words associated with particular themes, do both (Though they could both be in the same book, just check up as my memory on this matter fails me).

3. Powerprep software provided by ETS themselves for practice exams Taking practice exams are the most crucial part of your preparations. Take them seriously and take the complete tests with 5 sections, with properly timed breaks. I am personally a big believer in the practice tests and honestly believe that if a personal diligently and sincerely does the practice tests then he/she will perform exceedingly well in the actual exams. As I mentioned there are no shortcuts to preparing for GRE and as clichéd as this will sound do one of the powerprep exams before you start preparing and one as the last exam that you do before you write the actual GRE test. In between make use of the following sources of practice tests: a. Kaplan 5 online tests b. Barrons CD (2 tests) c. Princeton tests (how many ever there are)
Now I would suggest start with the Barrons or Princeton and then go the other. After which, you move on to the 5 Kaplan tests because they are proper 5 section tests. And then finally the last should the powerprep software test, as they will give you the best match to the type of questions that can be expected in your exam.
The reason I suggest you write these many tests because a very big part of the test is to prepared for utilizing your brain at high efficiency for 4 hours, which is not an easy task. So, the only way to make sure you can do that is to replicate the same conditions during your practice tests. So, you can be attentive and deliver your best for all the 4 hours.
Dos: a. After finishing each practice test go back and review each answer, the correct ones and the wrong ones. The correct ones because you never know they might have had a different reasoning or might have a different method of solving the problem. And the more you know the better. The wrong ones because of obvious reasons. Now apart from this, what else you are expected to do is to go through ever word in the exam and see if you know the meaning if you don’t then note is done and look it up later. Make a note of the sections you are struggling with and then go back and practice those sections more before the next practice test. The particular type of Quant Problems and all of that. Writing a practice test is a 1 day arduous process. 3 hours of test because you will not do the AWA and then another at least 90 mins to thoroughly go through the answers properly and see where you messed up. And then following it up with more practice of those sections on the same day and all that is necessary. b. TAKE THE BREAKS AS SCHEDULED BY GRE. Because you need to give your brain timely rest and you also need to mentally and physically refresh. c. If you have scheduled your exam for the morning then your practice tests should be in the same hours. If it’s for the afternoon then take your practice tests in the afternoon. d. Eat and drink something during the breaks. This may seem as madness but remember your body needs to be used to performing at high levels at the time of the day. For example, I am used to sleeping in the afternoon and I booked my exam for the afternoon slot so because my body’s clock is telling me sleep at 2 PM I found it very tough to concentrate during the exam.
e. Try finishing each section with at least 5 to 10 mins to spare. And then go through the entire section properly again. There are always mistakes. f. Leave the questions you do not know for the last. Come back to them after you have finished the entire section otherwise.
Don’ts: a. Over strain your brains by not taking the breaks and working it continuously, I know of people who have done it and failed miserably because they just weren’t able to sit for 4 hours so they wanted to leave as soon as possible. Biggest mistake ever! Let the superheroes do the superhuman deeds. Remember you are a human. b. Just look at your final score and if you have done well just leave it. Go through the entire process I have aforementioned. c. Spend more than 2 mins on a question. Come back to questions if you do not know them but 1st finish the questions you know quickly.


4. A few of the Kaplan online exams The same explanation as above mentioned. Just remember the best way to go about things to do an example at the beginning and doing an exam at the end of every week and towards the end the last 3 or 4 days everyday one exam.

5. The Magoosh GRE website (For practising different difficulty level of Math and Verbal) Why am I mentioning the Magoosh website is very simple, you can select the difficult level of the questions you want to answer and how many questions you want to answer and what type of questions you want to answer and also the time frame within which you want to answer them. The best way to use this according to me is once you have finished at least 2 books of the 3 that I suggest you do. You get onto this website and take the hard difficulty questions of all the sections and answer the same amount of questions as asked in the actual exam but with 10 mins lesser than what will be available in the actual exam. This is just a beauty. I am aware of only this site which does this but I am sure there are others. Find them pay for them if required and use them to the optimum level.

6. The Official Guide I used the OG just for the purpose of reading the AWA examples and get a feel of what they expect. Though the true potential of the OG is that is gives you loads of practice problems to work with.
The Preparation: I prepared for one month and tried doing as much as possible. This was not enough as my results were below satisfaction. I worked with the Barrons book and the word power made easy book side by side. And did one exam at the end of every week and towards the end, maybe the last 3 or 4 days, I did an exam every day. I did not manage to finish the Barrons book or the word power made easy book properly.
But that was me, and now in retrospect, I realized my mistake and I’ve realized what was needed of me. So take it from a guy who has been there done that and failed miserably, there is NO shortcut for preparation. The more practice you do the better it will be for oneself. You need a minimum of 2 months of preparation and more than 2 months for learning your vocabulary.

Conclusion:

This is what I would suggest as a final word. Do what I mentioned about few people may call me paranoid and over the top but if you are the diligent kind and not the smartest bear in the forest then this is your key to success. Take a Banana or Red Bull or something that will revitalize you (maybe even an energy bar) and use it during the breaks. Study diligently and you are bound to get a 320 above definitely if not a 330 above.
This story might sound clichéd, over the top, and over doing it. You will hear a lot of people say that they did half of this and scored a 330 or a 320 above. While I am no one to refute their claims
because I have myself been witness to people doing really well with much lesser, I suggest, as a personal belief, this to be a sure road to success because this I am suggesting from my failures and from talking to a lot of people and asking them what they did.
I hope this overly long and boring brief helps you succeed. May the force be with you!