The first time I took the ASTB in December, I did not study at all and scored a 4 / 5 / 5 - 36 (pretty crappy). Then I retook it on August 1st. I studied for pretty much one month straight, about an hour a day using every version of the Arco I could find (I think 5th, 6th, and 7th additions), the marine study guide you see floating around here, and another study guide much like the Arco that I found at Barnes and Nobles. I studied for like a week, then took one of the practice tests, studied for another couple days, then took a practice test, until I had completed all of the navy practice tests in the study guides. Make sure you time yourself when taking the practice tests so it is more practical and realistic. Also, after you take the practice tests, go back over the ones you get wrong!! Oh, and be sure to spend the most time studying the Math since that weighs the heaviest (especially if math isn't your strong point (like myself). After that preperation over the course of one month, my second test scores were 5 / 7 / 6 - 50. Not great, but quite an improvement, and now competetive with the rest of my package. Good luck, hope this helps some.
-Ken