RC is the most generic aspect of the GMAT, i.e. it is not test specific. There is only one way to improve your reading comprehension: read. If you have all grammar aspects covered, only reading will lead you further. I've researched that biography is the best genre to pick up the common language. Other than that, read the Economist
and Scientific American. However, their articles are very demanding. Don't pace yourself at start, go to the next article only after you have fully understood the present one. I do not mean to read one paper and check the progress - only after having understood significant amount of text the skills will improve, I mean 5-10-20 books. Get informed about skim reading. You shall find this interesting too: http://en.wikipedia.org/wiki/Lexile
If I answered your question with this post, use the motivating power of kudos!