I had a real problem with this overall. Just focusing with every amount of your mental capacity for hours upon hours can certainly wear a person down.
While in the middle of a section it isn't possible to partake in anything, but on breaks I was sure to eat my favorite high energy junk food. Not the healthiest stuff, but caffeine laden drinks with something like a Cliff Bar seem to help. Of course you don't want to develop a tummy ache and must drink water as well. I think I also had oranges I could tear into.
I also used my breaks to walk around and clear my brain. I didn't sit and wallow or chat with other test takes. I got up and got the blood pumping while eating the above mentioned tasty snacks.
Perhaps not the healthiest in the long run but it was the kick I needed on test day. My test site was a 2.5 hour drive away from me, so I had an early day. In the vehicle I pumped myself up with some fast tunes. On the drive home I was crashing a bit from the sugar and crap, but I felt great for the time which mattered.