My tips are to remember that the ACT is essentially a time trial (in contrast, the SAT trips people up by phrasing the questions in a confusing way). Your score will pretty much depend on whether you can finish all of the questions with the time you're given.
On the writing section, spend at least 2 minutes just calmly thinking about everything you're gonna write (especially the evidence you're going to use in your essay, since the logical organization of your essay is what they actually grade). If you can write an essay in 20 minutes, you can do it in 18 too.
On math, don't do unnecessary algebra. If you have a problem where you can figure out the answer backwards (i.e., plugging the multiple choice answers into a given formula), do that. You don't get bonus points for rigorously proving the algebraic solution.
Lastly, on science, absolutely do not read the preamble that's given before each section. The science section tests your ability to understand what a graph is saying (like whether x is increasing along with y, or whether x & y are correlated, etc). There will likely be only one question on the entire science section that relies on a background understanding of the concept being demonstrated, and it's usually extremely simple (IIRC, on my test it was asking for the real reason why water is sucked up into a beaker when a match is lit underneath it). Unless you stumble upon a question that can't be answered without it (which you won't) then don't bother reading it. I'm not convinced that it's possible to finish the entire science section if you actually try to read and absorb the background information in each section.
