It depends on whether you are a native american english speaker or not.
Native American English:
Focus on studying grammar. MGMAT has a great run down of grammar needed for the test in the SC guide book.
Non-native
If you are a non-native speaker, the highest leverage thing you can do with your time is spend it with a native american english tutor reviewing gmat problems together. I have worked with many private students on this, the thing that correlates best with verbal improvement is hours-spent-reviewing-problems-with-native-american-english-speaker. The bad news: you need to hire a tutor. The good news: you don't need a gmat tutor (!$200/hr) to do this, it can be done well with any reading or writing tutor (~$30/hr.) In fact, I often refer my private students to another tutor for this portion of their study. They'll do about 10hrs total with me (GMAT-specific), but maybe 20hrs with the other tutor (reading-specific).
