If you have gone through the various GMAT guide books you may have already done this, but people on this forum speak very highly of the
Manhattan GMAT SC guide. I also thought that it was pretty good at getting the basics down.
When doing practice questions, focus on quality over quantity. It is much more important to spend an entire day on one question to understand the answer than go through problems quickly *thinking* you understand the concepts.
Do not move on to the next question until you can prove why all the wrong answers are wrong and why the correct answer is correct.
Supplement your SC books with grammar and usage websites such as Bartlebys.com
Additionally, participate in the forum, there is a lot to learn here!
Some non native speakers may have some additional advice.
Good luck!