One thing I did differently than the replies above was read books. I had the same problem where I had trouble switching from math to verbal - my brain could not make the switch. On each of the tests, I could either do well in verbal or math, but I couldn't perform well on both during the same test. Therefore, I realized I needed to increase my mental endurance. I took a few weeks off from the gmat and began reading intense books for 4 - 5 hours at a time (books on physics, economics, etc.). As I was reading these books, I began to notice that my mind would wonder after a few hours. I realized this and began to force myself to read the words slowly in my head and continue to visualize what I was reading. After reading a few books in a couple of weeks, the gmat became much easier - a lot of the material just flowed better and stuck in my brain. (While doing this, your quant will suffer, so you must continue to stay fresh in both areas)
Additionally, I practiced with some of the LSAT CR and RC after completing a math section - this really strained my mind and eventually made the gmat a bit easier...
Lastly, I would suggest that you always do the AWA prior to taking the tests. This will force you to work on your mental endurance...
These steps took me from mid 600s to 700+... I got to a point where I knew that I understood the concepts, but I had trouble enduring the four hours. It's a lot of work, but don't ever give up and you will get there...