I found all of the materials I used helpful especially the
GMAT CLUB TEST Products. It was just too complex for what the GMAT will likely ask. I think if you're looking to become a GMAT instructor, then maybe you would need that level but if you're looking for 700+, it might freak you out unnecessarily. Oh, and buy the erasable notepad, I find it much easier to do calculations with pen and paper than on the notepad thing so I'm glad I practiced.
My advice for verbal:
Always read all of the answer choices. This doesn't just apply to sentence correction. My strategy for verbal is elimination 99.9% of the time. It often comes down to two choices, which I then read very carefully again and I figure out that one of them has a flaw so I pick the other. I think a lot of the correct answers, for reading comprehension especially, is not the "best" answer but the least wrong answer. Like, if the question is "what does this passage most strongly suggest", don't go looking for an answer that summarizes the point of the paragraph. The answer is almost always some random tangent that is technically true because it was stated in the passage but has very little relevance to the overall point of the passage. On the other hand, as you're reading the passage, always ask yourself "where is the author going with this? what I do expect him or her to say next?" because whether or not this is a question that comes up, it helps you gain the context to understand the information.
My other advice for verbal is to practice reading quickly but carefully. I think one of the reasons I did so well in verbal is because I can read very, very quickly after years of eschewing social interaction for Slate articles. But I did have to practice reading carefully. Missing one key word in a question could screw you. Particularly for quant but also true for verbal (where there are generally more words). Read the Wall Street Journal, Forbes, the Economist. Read things that you're interested in and especially read things you're really not interested in. If you come across a reading comprehension passage that you're intimidated by (e.g. some crazy scientific topic where every eighth word is a chemical compound), pretend to be super interested in the topic and read it like your sexy next-door neighbour is going to quiz you on it later.
In conclusion:
I truly believe that if you put in the time and effort, the GMAT will be the easiest part of your MBA application. Don't get discouraged, don't give up, and don't let the clock run out.