One tip is to be sure to take the breaks offered between sections. I always take the whole 10 minutes and go outside/run around/splash cold water on my face. I feel a lot better after doing that. Don't just sit in the waiting room of the testing center - that probably doesn't help much. Get some fresh air and get your blood pumping to clear your head for the next section.

For reading comp in particular, I'd advise taking some short notes on each passage if you have trouble concentrating. Giving yourself a task to accomplish while reading will likely help.

A trick I did was to envision the entire test as many 2 minute tests where after each question I would completely wipe my mind blank with regards to the last question. Why? Because there's nothing you can do about the last question or the question before the last question once you've finalized your answer choice. So each new question was a new 2 minute test and that let me focus/refocus/focus/refocus and I felt that kept my nerves down, which helped me focus even more.