In my GMAT prep my two main problem areas have been really hard data sufficiency questions and reading comprehension passages pertaining to science. As per someones advice (I'm not sure if it was on here or in a prep book, sorry I can't give credit where it is due) I went into the bookstore to find a magazine or journal that helps me become more familiar to science passages. I picked up the magazine Science News, really because of its cover, and it has helped quite a bit. The passages even seem to be written in a manner similar to those found on the GMAT. I hope this helps someone as well as it has me, and if I anyone has any advice on those toughest of tough data sufficiency questions, I'm all ears!