These tips have nothing to do with your knowledge of the topics covered in the ACT. In fact, what you are about to read can be applied to any test-taking situation including classroom exams.
1. HAVE A SNACK AVAILABLE. There is a scheduled break between Math and Reading and you should refuel at that point. Take the next week to experiment with the type of foods you should bring: nothing too filling that will put you to sleep. Avoid turkey and some legumes, for example, because of the tryptophan effect. Keeping in mind that you aren't allowed to eat DURING the timed portions of the test, I usually have raisins in my pocket in case I start to lose energy. They are quiet and nutritious and won't cause a deleterious sugar high. No candy, please.
2. STAY HYDRATED. The brain needs fluid to facilitate electrical and chemical exchanges that are the basis of thought. Water is good. Soda pop is not. Coffee is not. Try out "sports drinks" and "energy boosters" before assuming that they will have the desired effect.
3. SIT UP. In addition to fluids, the brain needs oxygen. Slumping over the desk is robbing your brain of its ability to function at full force. Deep breaths, a little stretching, and straightening your back can refresh you without using up precious minutes.
4. USE BOTH SIDES OF YOUR BRAIN. Use your nondominant hand to mark your place in a passage or chart while reviewing questions or bubbling on the scantron sheet. The activity will help your brain to use both concrete and abstract strengths, and having a finger on your place will avoid the need to search for where you left off when moving your eyes from the test to the answer sheet.
5. HAVE A DEFAULT ANSWER READY AND WAITING. Statistically, every alternative is used an equal number of times in a section. Since the ACT does not penalize for incorrect answers, it is to your benefit to fill in a bubble for every question even if you aren't sure which alternative is correct. "Educated guesses" are, of course, preferable, but if you just don't know or don't have time to address all of the questions before time runs out, bubble straight down the scantron. It doesn't matter which position you choose. I take the third alternative -- C -- for a totally arbitrary reason. If you're superstitious, pick a default that has some meaning; for instance, select the first position -- A -- if your middle name is Alex.