Haha, I was in the same position as you are.
I agree with franglish. Reading helps. A lot. Classic novels and magazines like the Economist really help.
(I used a website, actually, for vocab prep as well. The company's called StarCatcher, and I used some of its online vocab tools when I beta-tested for it. It helped me pinpoint the words I didn't retain, and I ended up getting an 800 on the cr section.
StarCatcher if you want to check it out.)
Don't worry too much, though. The sentence completion section is easier to improve on than the reading comprehension section.