I am no grammarian, however I was able to reach a point where I could get virtually every single SC correct, in under 45 seconds, by doing.... brace yourself.... 5~10K practice questions. That's an example of how to do it the HARD WAY.
If I could go back in time 16 months, I would buy three books: OG11
verbal supplement, and MGMAT SC
. Although I never actually read the latter, I have seen a few of its online explanations and was quite impressed. If you master those three, you should be fine (80%+). If you want to increase your accuracy rate even higher, then put on your miner's cap and start digging deep into the archives of this forum. You will be amazed at what you find buried 100 pages back.