Great thread and really good advice. I know I'm stating the obvious but some simple/common sense actions have helped me tremendously make it through track days completely focused and energized.
1. I stopped driving to events the morning of. Waking up at 2am, driving 3-5 hours to the track and rushing to get ready for 4-6 sessions a day is apparently detrimental to mental endurance. A good night's sleep is necessary for mental focus and clarity.
2. No car work during events (unless necessary). Even swapping out brakes, flushing fluids etc will dehydrate you fast out in the sun. If work is needed, I kick everybody out of the tent and park the car in the shade.
3. Go in with Plan A, have Plan B ready. I never go out on a session without a specific plan of attack. Sometimes I'm hunting for half a sec, some other times I'm testing lines, tire pressures and slip angles. I've found that I get mentally tired easily if I go out there and drive around aimlessly. Don't get me wrong it's still fun, but it's not focused driving. Sometimes Plan A is foiled: too much traffic, limp modes, wrong tire pressure, I don't just aim to "save" the session, I shoot for an alternate test that is not impacted by the current problem at hand.
4. Similar to the advice above, I use the straights to relax and ensure I'm not tense by lifting thumps then rest of fingers off the wheel, one hand at a time. Check gauges, take a good breath.
Most importantly, crack a couple of beers open at the end of the day and enjoy with friends and track buddies!