At risk of sounding promotional, the best help I got in terms of practice tests, was from the website called GREBIO. The take-home message from the booklet and the exam itself that I got was that in addition to (obviously) your knowledge, your problem-solving skills are tested here. So the grebio practice tests helped a lot in that respect, since they have a bunch of questions like that. But their summary ebook is a bit too "bullet-pointy" for me. It might be good for other though.