I've lost track of the numerous "road warrior" marathon drives I have done.
Here's my rules of thumb.  Usual disclaimers apply.  And, there's no
substitute for being in good physical condition and getting a good nights
rest.
1.  Keep the passenger compartment on the cool side.
2.  Drink plenty of water.  (Caffeine and sugar will cause you to crash after
the buzz wears off.)
3.  Keep the radio on, but avoid soothing music.
4.  Eat VERY LIGHT and avoid meats and "heavy" foods.  Stay a bit hungry.
5.  Stop frequently, get out, walk or jog around, take some deep breaths.
6.  Change your driving position.  If you have it, turn off the cruise
control every so often.
7.  Keep somebody elses taillights in your vision.  That really helps in fog.
8.  Open a window.
9.  Talk, even if only to yourself.
10.  I also find that crossing the center line or driving into a ditch at 70
mph always gives me a real adrenaline boost.  ;-)
Greg
86.5 Camper Syncro WLE
And some other faster cars
Spokane, WA