I gave my GMAT a few days ago and scored a 770 with around 2-3 months of preparation.
I started my preparation by first going through the different types of questions and attempting a few full length mock tests. You can find a list of free CATs here in this forum by just searching for it (sorry, i'm new here and so am not allowed to post urls)
I was able to get scores around the ~700 mark. These tests should give a decent indication of where you need improvement. For me, it was SC as a whole and certain specific types of questions in RC. In Quant, I was making silly mistakes or getting stuck not knowing how to approach some of the problems (number theory, geometry, etc)
If there isn't enough time, it is essential to get to know exactly what to focus on. Taking a few mocks and going through the mistakes will give that insight. While most courses would ask you not to skip anything, due to the lack of time, I had to be concentrate on only learning things that were essential. I also took one of the two official GmatPrep CATs after I was fairly confident. This is by far the best way to judge where you stand and what the common errors are.
At this point, I started to look at the what was needed for SC. I came across GmatPrepNow and Veritas Prep videos on Youtube ... GmatPrepNow has excellent content which did what I was looking for - pick out the grammar parts needed and explain them in a concise manner.
For practice, I used VeritasPrep's free online practice. The questions and solutions are very good and it is available for free. GmatPrepNow had good solved examples which I used for the SC practice to identify exactly what mistakes I was making. The video explanations drove the point better than the textual answers I had seen elsewhere.
I then took over 10 mocks! 4 official GmatPrep ones (it's paid but is completely worth every cent) and 6 veritas prep. I thought the official mocks were much closer to the real test than Veritas, especially when it came to harder problems.
I saw a 20-30 point improvement in the last 1 week. Here is what helped:
* Time Management - GmatPrepNow has a free video on this. I used it and could immediately see results.
* Some basic formulas in Math (especially Triangles, Counting, Last digit of powers, etc) - I used to derive these in the exam and get the problem right but I realized it matters a lot to save time on these and spend them in the harder problems that aren't as straightforward
*
Official Guide SCs - It is not just about solving them or going through the answers. It is very important to know what they are testing and why each option is wrong. I started removing the factor of time when practicing and broke down each SC and compared why I marked an option as not correct and whether that was the same reason provided by experts. With time, the accuracy and speed increase a lot once the patterns became clear.