I have a huge weakness in reading comp, CR, and Geometry....i figured i would do as man practice problems as possible, because i did not do that before. I've been using the OG and 800score.com's online material
I recommend taking the time to study the resources available on the GMAT Blog as well as the GMAT resource wiki.
I think that they key thing to improve your scores would be to create a study strategy that systematically addresses your weaknesses and sharpens those skills. It's easier said then done, but there are plenty of examples on how you can do this on the resources I cited.
Good luck.
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Also, because you are looking for such a large increase, and because you have been struggling a bit on your own, you may need to take a class or work with a private tutor. Something to think about.
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Stacey Koprince
GMAT Instructor
Director of Online Community Manhattan GMAT