I do ALOT of road trips both for work and personal adventure!
Rule #1 - if you are tired, stop and sleep. Sometimes 15 or 20 minutes of a nap is all you need to stay frosty. Limit your total driving time and don't exceed it. Sleep in the car if you have to, don't risk your life or anyone else's when you are driving.
Now about the keeping sharp. Great music is a must. Even better if you aren't embarassed to sing out loud.
No big meals, small or snacks only. Big meals make me really sleepy.
Stay curious along the way. If you see something of interest, stop, get out and take a look at it. I live in Oregon, the first time I saw a dead armadillo in Texas - I STOPPED! DANG! THAT WAS A WEIRD CRITTER!
Enjoy yourself. Find beauty in the country you pass through. It makes the miles go by.
angela