The single biggest problem that I have observed among ACT-takers is that they never have enough time to finish the entire Reading section. 40 questions in 35 minutes is a lot, and if you're a slow reader, then it can be deadly.
My advice: skip one of the passages.
If you know you generally hate Prose Fiction, plan to skip that passage; if Science is usually awful, skip Science, etc. If you don't have a preference, skim through the four when you first get the test and see which one looks least interesting.
Now I realize what you're thinking: how can I possibly get a decent score if I omit a quarter of the section?
Here's how: first, you're not going to omit it completely. You're going to pick a letter (A, B, C, D) and fill it in for every single answer for that passage. Statistically, you are guaranteed to get at least two questions right, usually three, and sometimes even four (although I wouldn't bet on the last one).
You now have approximately 11 minutes and 30 seconds to spend on the remaining three passages. If you can use that extra time to get, say, 9/10 questions correct on each one, that already gives you 27 points. Add three more points from the omitted section, and that gives you a raw score of 30, which is usually equivalent to about a scaled score of 27 -- not bad if you've been stuck at 23 or 24.
Now, let's say you have a fantastic test and get 10/10 right on the other three passages. That's a raw score of 30. Plus three points from the omitted test = raw score of 33 = scaled score of 30. That's right, a 30.
I will admit that this strategy can be a little risky -- if it backfires, you can end up with a much lower score than what you started with, and sometimes that does happen initially. But it's important to stick with it because eventually, I promise, it will pay off. Sometimes it's the only chance you have to seriously increase your Reading score.