I can't speak directly to the ACT, because the last time I took it was in seventh grade (...not exactly by choice, though. my parents made me "for the experience".)
But one strategy I found immensly helpful for the SAT reading sections (because I am one of the slowest readers I know) is to not bother reading the whole thing. Read the first and last sentences of each paragraph, then skim it if you didn't get the general meaning from the first/last sentences. Answer all the questions you can; if a question refers to a specific line number, read that line and the sentences that surround it. Questions about the tone of the passage almost always have an answer like, "objective evaluation" or "ironically whistful". "Bitter sarcasm" and things like that rarely are the right anger, because if they put a passage that emotionally charged in the test, it would always end up offending someone. Guess on anything you're still not sure about and move on: it's more important to get through all the passages than to get 100% correct on the first two. Hopefully you'll have time to go back and read passages more in depth to get the hardest questions at the end. If not, no big loss.
That may or may not help for the ACT, and may not even work for everyone (but it did for me).
More important advice, which has already been given but which cannot be emphasized enough:
-Get 7-9 hours of sleep; whatever it takes to make you relaxed and alert
-Plan on taking it again, no matter what; it relieves some pressure now, and you'll have more experience later
-At least look through a practice test, so you know what all the rules and question formats are. There's no reason to be reading the directions for the first time when you're actually testing
