After receiving a note saying that someone might be interested in how I got a 740 (Q44,V49), I hope I can provide guidance. I can't speak to the quant so much as the verbal, but I hope this helps someone somewhere.
I decided to take the GMAT, in order to go get an MBA to open up more doors. I am an expert on Middle East Politics and security, but the market for jobs in those roles is... shifty depending on oil prices and US policy. At the time, my father had a series of major heart attacks causing him to spend weeks in the hospital and I studied the GMAT as a way to distract myself in the hospitals and appointments, as well as while I was monitoring him at home.
I have never been amazing at math. If you give me enough time, I can solve any GMAT problem, but on a timer, I panic and overthink things. This happened at my GMAT, which caused me to get a 44 after I ran out of time on the last four questions.
Nevertheless, in the way that some people just "get" math, I get communication. I always knew how to structure a paper from a very young age and how to make a compelling case for my argument while avoiding potential strong objections. The verbal portion of the GMAT, at first, seemed daunting. It was designed to be confusing. However, one thing that I noticed relatively early in the game is that, in general, the test rewards people who like to read and have read enough that they can identify persuasive writing tropes.
So (please don't use this as an archetype, this is an example), in general, when an article starts with "Americans love eating hot dogs. In the past 10 years..." it's all but certain that, assuming the publication is being produced for adults, there will be a "However,... hot dogs are full of nitrates and contribute to obesity." I don't know how I know that, but I do and the answer is that I read constantly. I like reading and I was an obese child so books were definitely my friend.
The key to reading and writing is to understand how people have acculturated to writing. Verbal skills are pretty much culturally determined and subjective. The GMAT may have taken the English language and sanitized it to such an extent that they claim it has firm rules, but it's just one style of writing that may go out of fashion. One hundred and fifty years ago, the sentences I am writing now would be considered way too simple, because long, loquacious sentences were valued as as sign of literacy and class.
So, given that the GMAT is ultimately an aggregated subjective opinion about what is correct, you need to prepare for that. You can study all the idioms and grammatical expressions you want, but to really exceed at the GMAT verbal, particularly in the reading and critical reasoning sections, you need to be a reader. I believe it helps to know a foreign language. I speak Arabic so I think a lot about how grammar changes in certain contexts, which I think gives me a "meta" perspective on English.
Regardless, if you want to succeed on the GMAT verbal, it's not enough to study. You have to read and, to a certain extent, you have to like to read. Force yourself to read academic subjects' materials and you will be rewarded on the GMAT. I, for one, recommend Robert Caro's Master of the Senate or Lawrence in Arabia for academic books that interest me that force me to read for context and narrative. Longform articles in trade journals are good, too.
Nevertheless, I think it is key to note that the only reason I was likely able to get a 740 was because the score distribution, I believe (someone correct me if I am wrong), favors high quants and ok-ish verbals around the country. I am not a traditional MBA applicant and so I haven't been steeped in finance or math. I have made my career so far bullshitting and connecting numbers and data analysis to real world phenomena and adding the qualitative narrative. So, by inverting the usual GMAT applicant (high quant, low-ish verbal) to a high verbal and mediocre quant, i received a high score. so, if you think you're good at quant and you believe that an extra point or two will make the difference between a 680 and a 760, I would reconsider solely focusing on quant and focus on improving your verbal margins (assuming your target schools don't heavily weigh quant over verbal).