Because a decent breakfast is the nitrous oxide your brain craves in the morning.
I highly recommend something greasy and fattening. I know it sounds like bullshit, but just trust me.
Sausage is brain food.
ACT tests and SAT tests aren't that bad. Theres already been some good advice. A good nights rest does wonders as well. A good breakfast helps as well as going prepared. Oh, and I suggest not getting a nose bleed during the test. When I was taking the test some kid in front of me got a nose bleed. I felt bad for him since he was pretty much screwed.
I agree with Linki on that point. In fact, if you feel overwhelmed by a question and feel your time slipping away, just move on to the next question and put the previous one to the back of your mind. Save it for a second-run through the questions that you skipped/gave your trouble, if time allows.