This answer is based on some interesting post I found in Quora about the experience of Lawrence Choiu . I am reposting it here since I do find it quite interesting.

Preparing for the exam

First, you should start out as a decent test-taker. Pre-preparation, you'll want to start out scoring at least 325-330 (if not higher) on your first couple of practice exams before deciding to aim for a 340.* Additionally, it helps to have taken and done well on other exams written by ETS, such as the SAT, if only to have familiarized yourself with the general style of ETS's question writing.

Second, practice. Once you start consistently scoring at least 165-167 on each section, you have all the fundamentals down. You might want to brush up on a little bit of math or some vocabulary words that you're missing here and there, but otherwise it's practice all the way. Once you get up into the 336+ range, especially, the only thing that separates a 339-340 and a lower score is a couple mistakes. Consistency and practice are the difference-makers here.

For this reason, I would strongly recommend doing computerized practice questions only (and there are services that offer more practice questions in the official computerized format than you could possibly need). You want to have the comfort of "home-court advantage" when taking a test—if the home team gets an additional 3-4 points per game per college basketball game, then being intimately familiar with the computerized test format might be the difference maker between a 339 and a 340. By the end, I was really fast at clicking the bubbles, marking questions, and navigating through the test in general. It might seem trivial, but the advantage of "getting in the zone" (not to mention saving a few seconds per section) is priceless.

Test-day war plan

Make multiple full passes through each section. Strategy-wise, I took at least two passes through each quantitative reasoning section, doing the problems from scratch each time. With enough speed gained through practice, I was able to make a comfortable pass through each section in less than half the time allotted. It's definitely probable that most test-takers could get a 170 from doing just one careful pass (and maybe one check pass), but I felt that I maximized my chances (if only by a tiny bit) by doing the problems really quickly more than once.

On the first and second passes, I would mark a question (and come back on a second or third pass) if I was slightest bit unconfident about my answer. On the second pass, especially, try your best to ignore what you did on your first pass; furthermore, I would recommend starting from the end of the section (the harder questions) first. This is extremely crucial for catching the small mistakes or consistent mis-readings of questions that often create one or two mistakes on the quantitative reasoning section. I would save a third pass just as a general sanity check or for one or two questions that I marked from the first two passes. If I felt that time was running down, I'd start checking the marked questions halfway through the second pass.

Similarly, I would take at least two passes through each verbal reasoning section but wouldn't necessarily re-read the passages. Here, the emphasis was more on a couple of trickier questions (which I would mark during the first pass). It was harder for me to get a 170 on the verbal reasoning section but for a different reason: it was less about random mistakes and more about honing in on a few tricky questions. (A general strategy that applies to any test-taker is to shield out the answer choices when answering the passage-based questions, as the wrong answer choices only serve to distract—provide your own general response first before looking at and choosing among the answer choices.)

Take frequent breaks. Even if I felt really good about a section I had just taken, I would always opt to take at least a short breather after that section.

Pro tip: There's a long break in the middle of the test, but realistically you probably have enough time to drink water or use the restroom after any individual section. The ETS gives you a lot of time to read the general instructions at the beginning of each section (if I recall correctly, several minutes), and you can even click the button to request extra time to read the instructions, which you can do once per section. Honestly, unless you're fretting to begin the next section, I don't see any disadvantage to taking advantage of that time to "reset" and clear your mind before each section, especially if the previous section was a bit more hectic than you would have liked.

Note that while some of the strategies I mention above are applicable to all test-takers, some are good only if you're looking to get a near-perfect scaled score (and otherwise could potentially be rather poor strategies if extreme accuracy is not your limiting factor, as you might be missing questions due to the rapid pace of making multiple passes).
*I'm not saying it's not possible to get a 340 starting out with a lower score, depending on exactly how and why you got the lower score, but if you're starting far below a 330, shooting for a 340 starts to become less realistic and a matter of diminishing marginal returns on your time: the GRE is less important to grad admissions than the MCAT and SAT are to medical school and college admissions, respectively. Realistically, anything above a 330 is quite sufficient for admission to most top grad schools in the U.S. if your other credentials are good, especially if you're applying as a domestic student (though do note that many top math/engineering/physics programs desire a near-170 quantitative reasoning score as a sanity check). Even if you're looking to get a score in the 336-340 range, note that the grind to get from, say, a 330 to a 335 is arguably faster and less treacherous than that to get from a 336 to a 340.

If you're dead-set on a 340, though, I'd start with the steps highlighted in the many excellent answers to How do I prepare for the GRE to score above 330?

There are certainly accounts of people out there who have put hundreds of hours over many months preparing for the GRE, which is certainly warranted in some cases depending on their goals, but putting in the extra month of study to improve just to a couple points is not really worth it. Do more research. Get more job experience. Learn more about the world. Eat some pie.