Improving PSAT Writing Score?

<p>I'm currently a sophomore in high school who's planning on taking the PSAT's in a week or two. </p>

<p>My most recent PSAT scores have ranged from:
M: 74-77
CR: 55 - 60 - 70
W: 65 - 66 - 67
(I lost the score to one of the Math sections)</p>

<p>I've been doing a lot of practice problems, but I feel like I'm not improving. I really want to get at least a 213 (The minimum for National Merit) this year so that I wont feel so stressed to do well next year. </p>

<p>My Math is at its limit and CR is just painfully hard for me, so i'm depending mainly on luck for that. The only think that I really NEED to increase, is writing.</p>

<p>Are there any guides or books out there that can help me increase my writing? (And is there a REALLY helpful guide for CR?)</p>