A great question! It is true the GMAT (and the GRE) modify articles to make them more challenging. By only reading WSJ/Economist
you can only prep for the GMAT up until a point.
One option is to read even more challenging pieces. Both the New York Times Book Review and the London Review of books offer challenging writing. http://www.nytimes.com/pages/books/review/index.htmlhttp://www.lrb.co.uk/
While the writing found here may be daunting, it does not have the formal structure of GMAT passages. So really there is nothing quite like the GMAT RC passage, except the LSAT passage. LSAT luckily offers dozens of former tests, which translates to hundreds of RC passages.
Practice both LSAT and GMAT passages and get used to the style. When necessary use a dictionary. But remember, with any of these passages you always want to keep in mind the big picture, and not get lost in the welter of details.
Please feel free to ask me any other questions you may have.
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