I have Kaplan's book and yes, it helps a little, but I guess I should take some time to actually analyze my bad choices more than trying to figure out how to answer correctly. I'm usually pretty close most of the time - for example, I eliminate 3 answers and I pick the wrong one out of the remaining two. That's so frustrating: most of my wrong answers are picking the wrong one out of two. So I guess I'm doing something right, but it's not enough.
I think at this point I should focus more on my wrong answers and finding my own formula for answering correctly. For example, Kaplan (or was it Princeton?) suggests that we should read the question only, try to figure the answer on our own and only then find the matching one. This isn't working for me AT ALL, because I'm influenced by my own understanding of the text even before I look at the answers.
This is good advice, especially for difficult passages about a subject you know nothing about.
Strangely (?) enough, I'm usually better with texts outside my field. The less I know about the actual subject, the better.