There are some good books out there that are kind of like "1000 most tested words" or something along that line. I mean, you won't find it marketed for the PCAT but maybe for MCAT or GRE. Those are helpful... it's a little much to literally memorize them all but at least you've seen more words and will absorb some of them!
Also, learning roots can be helpful but I have found that for me personally there just seems to be too many exceptions. But anything helps.
For those of you who are not native English speakers, I would think just reading as many scientific articles (National Geographic, Scientific American, Medical Section of New York Post on Tuesdays) as possible would familiarize you with English language and the way it flows. It's hard to learn that from a book.
But don't worry too much. I think that when schools see that you do not speak English as your first language, they're a little more understanding with your verbal and Reading Comprehension scores.