I used meditation / breathing exercises. (I know it sounds nuts, but man did it work for me.)
During the 8 minute break, I make sure to spend at least 60 seconds with my eyes closed freeing my mind of any connection to the GMAT. I, instead focus on breathing. My favorite cadence is 3 seconds in, 3 seconds hold, 6 seconds out. That's 5 breaths per minute. I challenge you to close your eyes, take 5 of those breaths, and not become more relaxed.
(NB, the timing on the breaths seems to matter. I don't know if mine is ideal, but it works and other rhythms don't work.)
Also, I actually used this technique DURING the test. I have the exact same challenge you do, but I hit the wall after about 45 minutes. So, I use this technique in the middle of the section. Meaning, I only use 74 of my 75 minutes to answer questions. I have found that the tradeoff is worth it for me.
I probably do this about 4 times during the whole test.