How can I improve my PSAT scores? And is my score decent?

I just recieved my PSAT scores today (I'm a sopre) and I was wondering if a 1640 was a good score? I got a 600 on the reading, 550 on the math, and 490 on the writing. If thats not good i want to know, because i want constructive criticism on how i can improve. I plan on taking them again next October as a Junior, and the SAT in the spring of 2011.