Crash course in brain surgery;
Go to an pilot-supply/aviation supply store. Buy a private pilot course guide. Better yet, buy the FAA "Pilot's Handbook of Aeronautical Knowledge". Reference number FAA-H-8083-25. This will do wonder's for your aeronautical knowledge if you learn it, and picking up a Private Pilot Written Test Bank book (available from ASA or Jeppesen) will also help with the aeronautical questions. Learn these books, know these books. It will take at least weeks if you hit it hard, and you need to hit it hard. Don't re-take the ASTB untill you are ready, in other words do it on YOUR terms.
Physics, we are talking either 12th grade or college freshman level, maybe go buy both books and get familer with topics and problems involving; pulleys, torque, balancing things like see-saws, hydraulic pressure, force vectors, mechanical advantage, levers, ramps, gears, and any other major topics. The test does NOT go into depth with any of these topics, but you should know the basic principles behind each of them and be able to do simple problems involving them.
Math, this is the kicker. I haven't had "word problems" since sometime in high school, even though I am a college grad. There was no time spent on such questions, and I felt that these were the most challenging part of the test. Someone suggested a book that focuses ENTIRELY on word questions. IMO that would be a great way to go, and the other math such as BASIC trig, powers, and basic computation, is not very hard. Study up on trig a little, but you are not going to have to do sin, cos, etc, what you need to know are things like area of triangle, area of circle, circumfrance of circle, supplumentry angles, complentry angles, etc.
Nautical knowledge. I don't know a good place to look for this. My dad was a senior cheif and we had a boat and went boating a lot. You learn a lot by absorption in that enviroment. Some basic sailing/nautical books might be good. The glossary would be extremely helpful in such a book. Know all the parts of a boat. Know which way a rudder turns a boat (hint, exactly the same as an airplane).
Spacial Appreciation. There are things that I teach as a flight instructor that exactly parallel the spacial appreciation. Draw a huge cross-hair in the picture in the spacial apprectiation section (but don't really because you aren't supposed to write in the book, but on the other hand you can always erase or just mark a light dot in the center) on the picture, with the horizontal and vertical lines parallel with the PICTURE and not the ground.
In other words it will look like a "window" now with huge cross hairs. Now, you can determine if the airplane is climbing, if it is level, or if it is diving. If the horizen line is below that dot in the center, it's climbing. If it is on the dot, it is level regardless of the bank. If it is above then the aircraft is diving. Now, as you do this (it only takes a few seconds) say out loud "the aircraft is banking to the right, and diving, and heading out to sea (based on the picture)". Now that you have verbalized what is going on, it will be easy to pick out the picture that matches those 3 criteria. It is a little difficult to describe what I mean through words, but hopefully you can get the "idea". What I am doing is giving you a method that will always work. Determine the 3 factors by making "crosshairs", either for real or imaginary, and then look for those 3 factors in the possabilities.
Reading Comprehension, again this is a hard one to nail down, as it seems that reading comprehension is something that is gradually learned and not picked up very fast. I'm sure studying reading comprehension questions will help though. If it helps, on the form that I took of the ASTB (form 5) the ONLY reading comprehension questions asked you to "select the true statement based on the paragraph above". Those were the ONLY reading/verbal skill questions in the test.
Most universities/colleges allow you to retake courses for better grades. That could help big time if you are at all capable of doing so. Shooting to raise your GPA to 3+ would be a huge improvement. 3.anything sounds infinitely better than 2.something.
So, what you have to do is become resourcefull. There are a few resources out there that have the information you need. The amount of studying you do is going to be fairly excessive compared to the amount of critical information you are going to get out of each of these areas, but remember that the information is CRITICAL, and your score is CRITICAL, so the future of your life is WORTH studing your arse off for a few weeks or months to pass the test with competative scores. Anything worth doing is worth doing well. Life in the military is competative, so get used to putting out the effort, even if it seems far beyond what you thought you were capable of at first.