I had a 34 composite on mine, so I'll tell you what *I* think helped me (it worked for me, but that doesn't mean it works for everyone).
Math:
Really, there's nothing you can do to prepare for the math section except do math problems. You probably know you should be comfortable with trigonometry, you should know how to manipulate equations, geometry, maybe a little bit of high school probability. Definitely work practice tests for these. If you don't immediately know how to begin working a problem, then skip it and come back. You have exactly one minute per problem, and sometimes they're easier to do after you've warmed up.
Reading:
Reading is INCREDIBLY straightforward. You just have to read quickly while taking in as much information as you can. Best you can do is practice reading quickly. 99% of the time, the answers come word for word from the passage. It helps to keep tabs in your mind on what each paragraph was about.
Science:
This is similar to reading. No scientific knowledge is actually necessary, so treat it like reading in which you're required to be able to interpret graphs and scientific arguments. If you're posting on physicsforums, chances are you're aquainted with some science.
English:
You have to make sure you know your grammar. Know exactly when you use semicolons. Know exactly when to use commas, apostrophes, quotation marks. Know how to identify fragments and run-ons. Best thing you can do is review english lessons online.
And of course, the absolute best thing to do is to take a practice ACT. Actually, take as many as you can get your hands on. Take it exactly like the testing conditions. I took 5 practice tests before I made the 34 (my second attempt). On the last two, I actually gave myself LESS time than the test allows.
