As previous posters have mentioned, the ACT is all about time. You should probably bring a watch or make sure that your seat is close to a clock so you can pace yourself throughout the entire test. The Real ACT prep book is the best one, because some of the other companies have practice tests of questionable similarity to the actual thing.
For the science section, I would recommend skimming over the questions first and going back to find the answers within the text instead of vice versa. The science passages have a vast amount of information in them that you will not even need to answer the questions; hence, reading them would essentially be a waste of time. This wouldn't necessarily work for everyone though. It depends on how good you are at pinpointing where to find an answer in a table/chart/etc.
For the reading section, if I remember correctly, there are like two more technical passages along with two more humanities/literary passages. The literary passages are more like the SAT (analyzing themes/less fact-based) while the technical passages are more about facts (what is mentioned in the passage versus what isn't). In this section, start with the passages that you are naturally better at so that you give yourself more time for the ones you may have more trouble with. For me, I started out with the technical ones and sped through them so that I had extra time to deal with and think about the humanities passages.
Hope it helps! These tips really helped me score very well on my ACT. Feel free to PM me if you have any additional questions.
