Thanks sam2304 and bharathS.. I understand that verbal is the biggest challenge for non-native speakers. However, the verbal tested on the gmat can be learned. GMAT tests only certain concepts.
To start with, SC has a very high scope of improvement. I did MGMAT SC guide. It has got, probably, all the grammar rules that are tested on the gmat.you should complete this book 3-4 times. Believe me, you will be able to extract something new, everytime you revise this book. But one serious piece of advice: please allow yourself sometime so that these SC Concepts get absorbed in your mind. By the time u have covered all your verbal theories, these concepts should become secondary in nature for you. The SC errors will be evident, most of the time, automatically.
CR
Initially, I thought that there's some kind of logic for every possible CR question. But, it is not so. Relying on formal logics for solving CR questions is very dangerous. Instead, focus on getting the right meaning of the argument. Prephase it. Then come up with your answer. This might take you to the right answer in majority of the cases. However, there are certain logical flaws that can be relied upon such as CAUSE and EFFECT, causality, and wrong interpretation of statistics etc. Do as many problems as you can. But again, this is the biggest discovery in my entire GMAT preparation. No other testprep company's CR is close to GMAT CR. GMAT employs very clear logic that is understandable. Their CR questions do not confuse the reader. So rely on OG-12, OG VERBAL, and GMATPREP Tests for the actual thing.
RC:
Do not get into the passage. You have to figure out the structure of the passage. However, while reading, you should make note of what-things-appear-where in the passage. Figure out the logic of the RC. Atlast, you should be able to tell 'what and why is the author saying this'.. Many RC questions can be answered based on your understanding of the logic of the RC. Just following this may result in great improvement. I must tell you that I did not read wallstreet journal or economic times. I am neither a voracious reader. I think that RC is really about getting the meaning and the logic of the passage.
Hope this helps.