As a person that drives 540 miles just about every other weekend the best advice I can offer is find a rest stop and pull over for a few minutes.

I've had days where, no matter what, I couldn't shake feeling tired. I'd pull over in a safe area, take a 15-20 minute nap, and I was good to go. Often enough, just pulling over and taking a nice stretch was all I needed.

I've tried caffeinated drinks, cold air, hot air, eating, not eating, blasting the radio, basically everything I can think of. Still, for me, nothing works better than taking a little nap.