This is my ACT advice. I got a 36 so trust me. To be honest this isn't exactly what I did; this is what I should have done. I skimmed through a lot of this but if I had spent more time on it, I wouldn't have had to worry about it as much and would have had ALL MY BASES COVERED.
I spent tons of times on forums and things and found this to be the perfect combination of books and advice. So without further ado:
1. Read the following guides. They aren't too long and you don't have to absorb everything word for word. Just skim through and look closer at the things you don't know how to do. Jeandevaches' Guide to the ACT ACT Tips to Succeed
2. Take practice test(s) from The Real Act (red book). It contains official ACT's released and is the best bet to predict your scores and practice from.*
3. Read the Barron's ACT 36 book. It focuses on the higher scores and offers helpful advice, instead of making you wade through pages and pages of useless stuff and having one or two helpful facts like most prep books. I found the English sections especially helpful and if you don't have time, read just those parts.
4. Read the math sections of the Gruber's ACT book and do practice problems from there. It has a pretty solid math section and covers the important things you need to know.
* Note: It is a good idea to intermingle tests in between these steps. Try taking tests after each step and seeing how you improve. Try your best to take the full test in one sitting without many breaks, even if you have to build up to this goal. If you don't, the scores you get will probably not be accurate. Take as many practice tests as you can, its the single best way to improve your score. But always REVIEW your results.
A lot of test taking comes down to natural talent. Some people are good at certain tests and some people aren't. I'm sure you have heard this B.S. before. A lot of the testing covered by the ACT is easy for some people because they have been learning the same type of stuff in class from day 1. They are able to take a few practice tests and luck into a high score. But to virtually ensure a high score, in the 33+ range, you can follow these steps. I assure you they will work and you will be satisfied. Money back guarantee