SC is hard to master in a short period of time because it's based on a solid knowledge of English grammar rules, and those can take time to really absorb and be able to implement. See if your local library or bookstore has a copy of Grammar Smart or Strunk's The Elements of Style. Both are excellent grammar resources and should help you greatly. Then just practice with as many SC questions as you can.
What other practice materials do you have besides Kaplan adn GMAT Plus? You should be going through the Official Guide, and the Princeton Review book with CD (Cracking the GMAT) is also excellent practice. Actually, I think their Verbal Workout book covers a lot of grammar rules, the key ones you need to know for the GMAT, so you might be able to cover all the bases just by picking up that book.