I will try to be short.
I took a 750 (Q49, V44); the first and second official prep test were respectively 650 (Q48, V31) and 680 (Q42, V41). As you can see, my Q and V section went back and forth in a very strange way. In the first attempt i studied almost only for the quant section, and i was just started to study verbal part actually. I never had big trouble with verbal and, what is more strange, my performance in verbal was worst when i started to study than it does when i was completely illiterate on that (in my first practice test i take a good V37 and i barely known what critical reasoning and sentence correction were).
Almost 4 months has passed between the 650 and the 680, and i do i studied, but not so much, mainly to recover my verbal skills; only 20 days has passed between 680 and 750, and i've simply didn't study at all (4 o 5 hours at max, and almost all just after the 680 test, because i was very angry with myself and i wanted to discover what didn't work), simply because i had a lot of work to do.
You guys have to take into account that my better score in practice test was only 710 (moreover, in an official practice test), so simply i can't explain how i achieve this result.
My native language is italian (and maybe you don't know, but italians are the worst in english) and i dedicated very few hours to verbal (let's say something like 25 hours), but i think this way is ok, because go into extreme details as i saw also here in gmat club is completely pointless (unless, of course, you want a 800).
I spent a lot of time in math instead, and i realized that algebra was my weakness, that's weird because i'm an engineer.
Indeed, even if i'm used to math, i have always been better with words than with numbers, so maybe the little amount of hours in verbal study is just my thing.
Some advice:
1) don't waste time searching perfection in math: the additional effort from 49 to 51 is huge and bring with him only 10 or 20 additional points (depending on verbal score -see gmat score table to realize it-)
2) use the super-tough math question in gmat club wisely: they might feel you like an idiot (when you actually are not); moreover, keep remember to yourself that actual Gmat questions are much easier
3) about verbal, honestly i can't figure out why this section seems to be so impossible to the people. For critical reasoning, i suggest to study formal logic. Everyone, including GMAC says "there's no need to know formal logic to do well in CR"; maybe it's true, but obviously the CR passages, to be reliable, have to be logically consistent, so have to follow rules of formal logic, like implication and so on. Moreover, in CR read the question before the passage, so you know what you are searching for while you read.
For reading comp., i suggest to read the first question first, and then read the all passage, this let you know what you have to search to answer first question and give to your mind a brief of the entire passage that will help you in the following questions.
For sentence correction, it's useless to study tons of little rules, because almost everytime the problem is not a little rule, but a flaw in the logic due to a grammatical error; so it's all about to think what the sentence means, not to find the little flaw. Obviously the most important rule have to be learned (subject verb agreement, parallelism and the others categories you find for instance in
Manhattan GMAT)
I don't like to advise about what to use or not to prepare, but, speaking about free stuff, the Veritas prep videos on youtube has been maybe the most important resources for me: few hours in which you get the right forma mentis.
That's all!