Get the Marine ASTB Gouge - The one with the Hornet on the cover. That one is invaluable. I'd also consider picking up the ARCO book, its got a lot of good practice questions. Both of these have been mentioned.
In addition, I'd browse boating rules and nomenclature. I distinctly recall answering questions about these topics. If I remember correctly, there were also a one or two questions about the space program and Naval aircraft.
The best thing you can probably do, however, is to get proficient at the spatial apperception portion. Playing around with flight sims can help, but get to the point where you can answer them accurately in about 10 seconds per question. It helped me to devise a system in which I first determined attitude - climbing, descending, level, then angle of bank - left, zero, right, and finally heading - out to sea, towards land, parallel.
Best of luck.