Advice from someone who fell asleep during two practice exams, but was able to focus on the real thing:
Get a good night's sleep.
Wake up early enough that you are not rushing to make it out the door.
If you are a coffee drinker, drink at least a little before the exam, but not so much that you have to pee a lot. On that note, use the restroom before starting the exam.
Eat breakfast.
If you have something that gets you pumped up/focused, do that. Listen to music, do a few jumping jacks, give yourself a pep talk, whatever.
I found it very helpful to stop before each new passage, close my eyes, take a deep breath, and clear my head. It only takes a few seconds, but it helped me forget the last passage and focus on the next one.
I'm not sure about anyone else, but it was 10x easier for me to focus on the actual exam than it was on any practice exam. I was psyched up because it was the real deal. However, I did have to force myself not to rush through psych, because I was so excited to be DONE.