Based on my GMAT experience, here are some tips:

Preparation time: This varies from person to person. If you can spend lot of time on preparation then 2 months should be sufficient. Else if you don’t leave your office till late in the evening like I do, 3-4 months should suffice. It is a good idea to start earlier and have look at the questions, lessons and [tag]GMAT[/tag] structure and gradually pick up steam to reach your peak on the GMAT day.

Material– Official Guide (OG) is a must combined with either Kaplan or Princeton. I used OG (for the quantitative section) and Kaplan (for Verbal). You can use the lessons and strategies from Kaplan and practice OG questions. I used Kaplan and the lessons were really helpful

By section
Quantitative:“ The quantitative skills tested on GMAT are pretty basic. Most of the Math required is already taught in school curriculum and requires simple formulas. Solve each question and then look at the answers. Books do recommend you to use elimination and other techniques to save time. However, I think anyone from India or engineering background should be able to solve each and every problem with time to spare. If you are reasonably good in Math, only place you can lose points is data sufficiency. Sometimes you see a question, assume something and jump to conclusion. You can go wrong there. For example, there might be 2 linear equations and you have been asked if to find value of one variable. It might seem that you can use both equations to get a unique solution but if you look carefully, both equations might reduce to similar forms.

If you not that strong quantitatively, start your preparation early and read all reference material on math once or twice then practice lots of questions from OG. Lessons are not as important as Math reference if you slightly weaker in this section. If you get your basics right you can easily do well in this section.

Verbal:“ I had a notion that your verbal score is totally based on verbal skills that you have developed over the years and it is not possible to improve it significantly. By the time I wrote GMAT I knew this was not the case. GMAT verbal follows specific patterns and answers are arrived at following certain rules. Of course your score depends on your verbal abilities but you can improve them significantly. That is where I missed out.

First thing is to make sure that you take the lessons for each of the 3 sub-sections. Kaplan lessons are great. Take them more than once till you remember all the points mentioned on the lessons.

Sentence correction:“ From lessons you will find that there are only few variations of grammatical mistakes tested on the GMAT. Lessons takes you through each of them and there tells you how to spot them. Keep that in mind and apply them without exception while practicing questions. Make sure to practice enough, such that spotting errors comes naturally. Finding the correct answer from here on should not be difficult.

Reading Comprehension:“ Again, lessons help a lot. When you read that passage make sure to think critically and question what’s the point, what does the author want to convey. This way you would understand what the author is trying to Imply and get the context and scope. Most RC questions on GMAT are based on these. Also for some questions, wrong answer choice might look more in scope and context of the actual passage i.e. by directly referring material in the passage. If you question these choices by “Is this really what author wanted to convey?” you can find flaws in these choices.

Critical reasoning:“ For this read the question before the text so that you know what you are looking for. You need to concentrate hard and spend some time on these questions.

Overall, the 3 most important factors that determine your final score are “ Focus, hard work and self-belief.