Read a book, preferably one with a technical or political viewpoint. It only takes a few hours, and it will condition your brain to analyze arguments, as you'll be subconsciously thinking about what the author is arguing, how compelling their argument is, what would make the argument more or less compelling, etc, all things you'll have to tackle on the GMAT. Since they're professionally edited, you'll also be looking at a lot of properly constructed sentences, and especially if English is a second language, you'll pick up the nuance of sentence structure, grammar, etc. Good luck.