I agree with IJR -- active reading is the way to go.
Brush up your grammar skills first. Spend some time with a solid English grammar review book to review the parts of speech, sentence fragments and run-ons, and the basics of sentence construction. A "fun" book like Writer's Express or English Grammar for Dummies is a great resource to have.
Memorize your idioms. Start applying the GMAT Idioms early in your studies and incorporate them into your everyday speech, emails, and English compositions. The more you can incorporate them into your English writing, the more confident you'll become. They may be slowly departing from the spotlight of GMAT SC, but I still suggest you study then!
Build your vocab with news sources. My recommendations include the New York Times, the Wall Street Journal, or any scholarly journal that you find interesting. Listen to NPR to keep up with the day's events and write down any new words you don't understand. Set a regular schedule for your reading and stick to it.
Make a vocab journal. Write down any words you don't know as you encounter them. You'll start to notice that certain words appear over and over again. Make flashcards for the ones that have tricky definitions or mean the opposite of what you'd expect.
Think like a thesaurus. It is much easier to memorize synonyms for words than their full definitions. Start grouping words together mentally (and on paper) according to their meaning. For example, words like "pusillanimous," "poltroonish," and "timorous" would go on the "shy" list.
Practice with harder Reading passages. Aiming for 700_? Once you've got the hang of how to take notes on GMAT Reading passages, seek out challenging passages to practice. If you can make sense of even the most dense, complex passages you'll likely find the GMAT Reading passages a cinch!
Take more time on GMAT Math word problems. Practice translating these questions from English keywords to Math equations. Be patient at first - these questions may be especially frustrating vocab-wise. Luckily, the common phrases such as "less than," "is the same as," and "product of" are easily memorized.
Look for prefixes, roots, and suffixes. If you don't know what a word means, try to break it into its component parts. Does it "sound" like a word you already know? It's likely that they have the same root!
Good luck!
