From what I've heard, the PSAT CR and Writing (besides the fact that there is no essay to be written on the PSAT) are pretty comparable to their SAT counterparts in terms of difficulty. The math on both tests differ slightly, with the SAT testing a bit of algebra II and pre-calculus. The PSAT math is pretty easy comparatively and I'm not even a math whiz like some of the guys here.
I'm studying for the PSAT's a bit this summer, here and there whenever I get the chance. I use SAT prep books and do the standard routine of reviewing. This PSAT prep is supposed to act as actual prep for the SAT's, hence the SAT prep books.