680 to 770 in ~6 weeks

Outside of very specific strategies pertaining to Sentence Correction and Critical Reasoning, jgordon257 has done a good job of summarizing our approach.

Of course, any attempt to summarize a test as complex as the GMAT will inevitably fall short. Despite the relatively small set of knowledge you need to memorize for the test, the GMAT is a highly difficult and competitive, world-class test of math, English, reasoning and writing skills. However, it can still be conquered through perseverance and the right strategy.

There is no "one size fits all" fix for someone with two months to study. You need to practice and discover your own weaknesses, then construct a study plan that fits you. For Jason, he just needed a crash course in SC and CR--he was already very solid in Quant.

Sentence Correction can get complicated, and has many facets. Thus, you will have to wait until my GMAT book for that one, but until then, the Meltzer book is an excellent substitute. For Critical Reasoning, we stuck to the core principles: 1) Identify your Conclusion, 2) Identify Your Evidence / Premise, 3) Predict Your Answer, 4) Eliminate Pre-Evidence, Out of Scope and Post-Conclusion Incorrect Answers, 5) Compare your Top 2 answers (if necessary) and 6) Verify correct answer. We also discussed visualization strategies to more fully understand each Critical Reasoning question type, and expanded on the five main categories of Assumptions (from "Ace the GMAT"): Cause and Effect, Comparison and Analogy, Representativeness, Implementation, and Questionable Evidence.

Part of why he succeeded is that he diligently completed (and reviewed!) every single question in the OG and QP1, as I asked him to do.

I am a strong believer that Full Practice Exams are overrated once a student has reached the high 600s/low 700s. At that point, it's more about obsessively working your way though every official question you can find (and reviewing them meticulously, making sure to note the reasons why each incorrect answer is incorrect). Outside strategy guides such as the Meltzer and Powerscore books, and GMAT Club Spreadsheets will also help with this process.

Especially when it comes to Verbal, it makes sense to practice with real GMAT questions only, since imitation Verbal questions inevitably fail to replicate the precise wording, unwritten rules and "feel" of real GMAT Verbal questions.

As far as study plans, strategies, learning resources, etc, you can check out My GMAT Action Plan (linked below in my signature). I have also released a free GMAT e-book / strategy guide of my own, as a publicly accessible Google document, which includes specific tips for Sentence Correction and Critical Reasoning.

That being said, there is also great value in discussing questions one-on-one, in real time, with a GMAT expert: being forced to verbalize your struggles often helps you to resolve them. This approach is hard to replicate without a tutor, but you could always join a study group, etc.. In addition, as Jason mentioned, there is value in accountability, and having a teacher, private tutor or "study buddy" who checks on your progress once a week.