Knowing when you loose focus is very important - you can plan you approach built on that. For myself I did the test in batches of 75-75-50 questions, with short breaks in between (not leaving the room, but closing my eyes for a couple of minutes, taking some deep breaths). After finishing the 200 I left the room for a drink of water and some snacks, returned and reviewed (note that leaving the room is very time consuming, since you have to do the full security check when returning).
When you have this rhythm you should practice full 4 hour practice exams as often as possible.
Concentration is key since the answer could hang on a single word in the sentence, so you have to keep concentration up over all 200 questions.