You should absolutely highlight but the key is getting comfortable with it. Even though I've matured to the point of not highlighting full sentences and just key words or phrases, when you first read the passage a lot of it looks useful!
You should highlight transitional words that indicate changes in the where the information is going. This will point out key arguments and contribute to your understand of the main idea. Understanding the main idea is key to accurate POE. If you like to check the question stems, you should also of course highlight words or phrases in the stems. I've noticed that the practice AAMC don't say "the word nature in paragraph 3 most likely means". Instead they just say "the word nature most likely..." and you have to go and frantically look for it.
Writing on the other hand is kind of risky since it takes a lot of time. TPR teaches you to START with writing things down while practicing and to eventually grow into doing it in your head.
I'm getting very close to doing well on the verbal passages, I just need to get over making foolish mistakes. My advice is to find a way to read the passage (there are various methods) that helps you retain the most information and get a good main idea within ~2 minutes. Then hit every question and ALWAYS go back to the passage. Don't just read the sentence or idea or word in question; read the surrounding paragraph.
