How should I prepare for the GRE?

Materials and Resources:

ETS Official GRE Guide (including the CD): You can obtain PDF online but I recommend that you actually buy this book for the PowerPrep tests CD that comes along with it. It is an extremely important tool given that it comes directly from the test takers i.e. ETS and will help you a lot in preparing your approach to the exam. The four tests (2 in the CD and 2 in the book) that come along with the book are very useful and I recommend that you finish each of them at least 2 days before the exam.
Barron's Guide to GRE: It is well and good if you have this book although I wouldn't say that it is a necessity. (PS: I borrowed it for a day from a friend and that's the only time I even used it)
Kaplan Practice Tests: If you have to buy the book to get the tests then please do so as it is worth the money. This tests were an integral part of my preparation and are extremely useful as they simulate actual GRE test conditions even though you take only one section at a time. I highly suggest that you finish the 20 sections (10 VR and 10 QA).
The Princeton Review Guide to GRE: This book is much similar, in terms of practice, to the other books I have mentioned but it had two or three important notes and points especially in the verbal section that helped me develop my strategy for the exam.
Magoosh Online GRE Prep - Although I would only say that it is extremely beneficial for you to peruse the site at least once, there is no denying that the Magoosh Flashcards app is a 'must-have' for all GRE test takers. It's innovative interface coupled with around 1000 important words make it extremely user friendly. Best part about it is, given it's an android app, I could use it absolutely anywhere; in the bus, on the elevator etc. I have even used it in boring lectures back in college. Don't have an Android phone? Fret not cause you can use these flashcards from your computer browser too. Magoosh GRE Vocabulary Flashcards
The Ubiquitous 400: If you ever feel burdened by the myriad of words in the Magoosh app or are too close to the exam and too many decks yet to be completed, use these flashcards. They are just a smaller version of Magoosh but are more focused on the high frequency words encountered in an average GRE test.
SparkNotes: GRE: This is a helpful site but I wasn't able to fully go through it and only studied the Essays section. From a very cursory glance though, it looks like a very helpful website on the whole and I can definitely vouch for the chapter on Essays which formed the crux of my preparation for the AWA section.

Out of the resources mentioned above, I would say that items 1 and 6 are necessities and the ETS Official GRE Guide cannot be done without.

Strategy and Tactics for QA:

Go through the Maths Review section in the ETS Official GRE Guide. Though it might seem like child's play at first glance, you may encounter something which you haven't encountered before. For example, I hadn't really studied much about the topic Quartiles in the Statistics section and would have been totally blindsided had I not come across it in the Maths Review section.
Once you are done with review, get going with the Kaplan Practice tests and finish them before moving on to the full length tests.
As has been mentioned before the quantitative section may look simple at the first glance, but some questions can be extremely tricky and pay attention to each and every single word, number or comparisons in the question. You don't want to be losing marks to silly mistakes here.
Please be extra careful in questions where a certain fact (like, X is a positive integer), comparison (0<c<d<1) or range of the variables (0<X<2) are mentioned. These are the questions where you have the maximum chances of making a mistake.
You will need at least around 10 minutes or so for verification. In most cases, you will be able to solve the question once more for verification and I recommend that, time permitting, you DO solve it once more.
Substituting values based on the ranges or inequalities provided in the question. It's a sure-shot way of confirming your answer.

Strategy and tactics for VR:

Go through the flashcard apps recommended and make sure you have memorized each and every word on all the decks. If you find yourself running short of time, use the Ubiquitous decks alone. Finish these decks by half a week before the exam and keep going through them in the remaining days before the exam lest you forget anything.
If you are having difficulty remembering certain words, I advocate writing them down in a small notepad or book and going through them more often than the other words. It may sound a trivial advice but it was of unbelievable help to me. Writing trumps reading when it comes to memorizing.
If you have more than a month or so before your exam, start reading the editorial sections in a newspaper or magazine and make sure you know each and every word's meaning thoroughly.
Attempt the Kaplan Practice tests and finish them before moving on to the full length tests.
Guessing words' meanings, though useful in certain scenarios, may be a BIG mistake in others, given the test's affinity to words which hold opposite meanings to what they actually sound like. For example, two high frequency words in the test are extant (approximate opposite of extinct) and equivocate (to mislead or lie).
Go through each and every option carefully and consider each of them, before you rule any out. This is especially true for the Reading Comprehension questions, which personally for me were the toughest part of the exam.
Some strategies like MOE (method of elimination) and trigger words (which I learnt about from The Princeton Review Guide to GRE) are important tools to have in your arsenal, in case you encounter a perplexing question.
From the POV of an Indian student, you should utilize the entire time allocated for these sections. In case you have finished answering all the questions, check and double-check each question until the time runs out.

Strategy and Tactics for AWA:

The Spark-notes site mentioned above turned out to be my savior as it was the only place from where I could a develop any sort of approach for this section. I advise that you go through it at least once and if possible more than once.
Do not skip this section while attempting the full length practice tests. This is the first section you will encounter in the test and can be the most taxing. Excluding this while doing the full length tests will leave you ill prepared for the main exam.
Practice! If you have had no prior experience of typing long essays or articles on the computer, you should at least write around 6-10 essays to get a feel of the section and improve your typing speed.
When unable to practice, read the sample essays in the various books mentioned above to understand what exactly the test takers are looking for i.e coherency in response, continuity of thought, and clear and unambiguous expression of ideas with ample support in terms of facts or examples.
Keep 5 minutes aside at the end for proof reading. You may have more ideas that you want to include but remember that even a small grammatical or spelling error makes a negative impression on the examiner. Better a short, organised and succinct essay rather than a long, haphazard and tedious one.

Some general and miscellaneous tips:
Adopt the three C's: Calmness, Composure and Confidence. Panic will be your biggest threat. Don't let it better you.
Keep an eye on time but don't attach too much attention to it. Focus on the question and you will be fine.
Use the mandatory 10 minute break wisely. Have something to eat, drink some water, use the washroom but more importantly, keep your mind completely empty.
GRE tests your endurance. Anything less than 7-8 hours of sleep on the previous day will not suffice.
In case, you can't find the answer for a particular question, Mark and Move On. Obsessing over the question is probably the worse thing that you can do.
Keep all documents prepared the day before and make sure you wear layered and comfortable clothing.

You have to buy the book and also practice the tests online...this can take 6 months...You have to practice as much as you can, it is never too much or enough for GRE, looking at the technicality of their questions..All the best!