I'll echo what everybody else is saying...
1) If you are not having problems with coming up with the right answer and are just slow at it, practice doing it in a very systematic way. Pitch, bank, direction...Pitch, bank, direction...over and over.
2) If you are having a problem just getting the right answer, do activities that force your mind to think in terms that allows you to come up with it. Taking flight lessons is a little pricey. Several hundred hours of flight simulator video games would be much more economical. I'm not kidding.
Once you get #2 down, work on #1.
And the disclaimer that I put in every thread about the ASTB like it or not- Some people aren't meant to be pilots or NFOs. The ASTB is an aptitude test for the qualities the Navy and Marine Corps want and if someone can't hack it, then it's not meant to be. No amount of studying can help some people.