Are you taking practice tests from GMATPrep and learning from your mistakes?
As you study OG - mark the questions you are getting wrong so you can review them religiously until you really master your mistakes. You can track your responses online here: http://www.gmatpill.com/official-guide
With your math plateau - make sure you understand variations of the same question. In other words, you might be asked a question about sequences for 1-100. But what if you were asked that same question but for 101-200? Would you fumble or would you have a general sense of how to approach the question.
These are the foundational concepts that you should understand. So it's helpful to do various word problems in which you modify one of the variables and get a different outcome. So to really make progress, make sure you understand your mistakes - but also what you would do if the question were worded slightly differently.
For a few examples on variations off the same base question - you can check out: http://www.gmatpill.com/gmat-practice-t ... -questions
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Zeke Lee, GMAT Pill Study Method (Study Less. Score More.)
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