Be well rested before starting.
Keep the temp coolish (for me that is the A/C cranked or just enough heat to be comfortable when cold out).
Listen to music or talk radio. Listening to something different can often be helpful.
I drink water except for when I need a pick me up. Then a diet pepsi or a diet mountain dew. Drink it all the time and it seems like it loses its punch.
Have some good snacks. Varies by who it is. For me, baby carrots and some ranch dip work well. Pretzels sometimes . Some hard candy or gum helps too. Setting a goal before I can have a snack or candy makes the miles pass too.
Take breaks every couple of hours. For me, the sweet spot for that is between 2.5-3 hours or so. Literally just 5-10 minutes out of the truck taking a bathroom break or walking does it for me. Usually enough to get me on to the next stretch. Gas stops don't work for me with a truck delivering 20+ mpg and a 36 gallon tank...
I know I need to take a break when I start fidgiting to stay alert and trying multiples of the tricks above. Being humble enough to admit its time to shut it down for a bit is key. I've found just taking a nap for even 15 minutes can make all the difference at times.
For me, a 12 hour drive is easily doable in one day. 15-16 hours if I am really pushing to make time, but it isn't fun by the end. That's for me as the only person in the car, driving the whole way. Add another driver, and the sky is the limit.