Agree that
MGMAT SC book will help you identify the types of SC errors to look out for. After that you need a lot of practice to hone your skills. If you've exhausted OG11 and OGVerbal Guide, start looking for fresh material: SC1000, Kaplan workout books, Princeton Review Verbal Guide, etc.
For CR, I've heard great things about Powerscore's CR Bible. I didn't use it only because I found I didn't tend to miss too many in CR so I focused my efforts on SC and RC. Give the CR1000 questions a shot to see how you fare.
For RC, Rhyme's approach is golden but it takes serious practice. I tried it and couldn't get it to work initially. Then I Rhyme chimed in on a thread and walked me (and others) through it. Since then, I tried the approach on several passages and now understand how to approach it.
Purchase challenges on this site. Math ones are tricky ricky but the verbal ones aren't too bad. If you feel you have a good grasp on concepts, the verbal challenges will provide you with new material to test your knowledge.
-JJ