Just finished the GMAT, scored 690. After 5 months of consistent prep, I'm done and happy with my score. Too many people on here bloat about how they score 730+. If you're from an overrepresented demographic then you may need that score, if that doesn't apply that anything over 650+ is suffice. There isn't that much difference between a Tier 1 school and a tier 2 school and the GMAT is only a small fraction of the application. Here's everything I would advise you to do.
Start you prep by taking an exam to get an understanding of where you are at. You could expect a 250 point increase from whatever that score is in 3-4 months of prep. Now time to get to work. I'll break down each section of the exam, ignore some of them if you're taking the revised exam.
Quant: You need to ensure you know all the concepts in math prior to practicing. Spend a few weeks on this. All test prep companies can help or self-study. None of the concepts are challenging and you likely need to refresh. After that practice a lot. Complete the OG Guides and Gmat Club for additional practice. Make sure for every problem you understand the concept used and how to apply. I used Gmat Ninja,
Target Test Prep and the Tested Tutor for help when I needed it. TTP is a very long course but also very good. If you really struggle, do the whole course, otherwise you can do the accelerated version or focus on areas you are weakest in.
Verbal:
Critical Reasoning: Watch the Gmat Ninja videos on this for strategy and then lots of practice. Not a whole to study here and mainly practice. Always identify the conclusion.
Reading Comprehension: Same as CR, watch Gmat Ninja and then lots of practice. Reading WSJ can also help. Make sure you understand the structure of the passage.
Sentence Correction: I really struggled with this one. Read three text books, completed TTP, Gmat Ninja, Thursdays with Ron and nothing helped. Eventually I just settled on getting parrelism, comparisons down and then using my ear. Got 70% accuracy in these which was fine for me.
Verbal is based on reasoning and reading ability, while quant is more understanding the concepts and applying. I think it is much easier to improve quant then verbal, if you have limited time, target the easy points.
Writing and IR: Didn't look at IR once and spent 2 hours reviewing the structure for writing.
The absolute most essential piece is the practice exams. You must take all six and review each question thoroughly. Aim to take at least six prior to the exam, and once you do that taking the exam will feel like a breeze. Other companies have there own versions, but nothing replicates the ones from GMAC. Those additional exams are optional.
The OG Guide is also the best source for practice questions and everyone should buy and complete it entirely.
Side note: In terms of courses, as many will likely agree TTP is the way to go. Even if you buy the six month subscription it is half the cost as the other companies and seemingly more effective.
Don't over analyze this exam. It's mainly a way to make some money on hopeful MBA applicants. Once you've spent 3-4 months preparing there is little more you can do.