I am a Junior when should I take the SAT's?

I am a Junior and I should probably be in the top tenth in my cl, if not a little higher. I want to take the SAT's more than once before I apply to colleges and I was hoping to apply to some places early. So when would be the best times to take the SAT's, and be able to do well on them? From the dates you give me when would be the best time to start preparing? Also I am a straight A student but on the PSAT's I only scored a 1690 but that was without preaparing too much. I had 640 in math but my reading and writing were only 510 and 540 respectively. Is that bad, and how can I do better?