Books on tape are great. Humor titles are my favorites, especially if I'm the only one awake.
(Laughing helps. Beware of tearjerkers; you don't want to be crying behind the wheel.)
One caution about using these in hot climates: tapes are vulnerable to heat, so it is best to store them in a little insulated cooler bag when you are not actually using them.
Don't drive without sunglasses in the daytime. Glare causes eyestrain, and you don't want that.
Do not eat heavy meals while on the road, and it's best not to eat too early, either, because then the night will seem longer. I always try stay away from fried foods, and also heavy starches such as pancakes. Think of it as the apres-Thanksgiving dinner effect. (Oh, turkey sandwiches are not an especially good idea; either, turkey makes you sleepy.)
If you are now on the shady side of 30 and you have not tried overnight driving lately, do not assume that you can still handle extensive night driving as well as you did in college. You probably can't.
If you wear contact lenses, use your spare glasses for the drive, or take your lenses out when you are riding as a passenger. It is harder for your eyes to focus in the dark, especially
when there are lights flashing by; contacts will often dry out and give you halo vision, which is exhausting and just plain dangerous. (For the same reason, point the vents away from your face, and keep wetting drops near at hand.)
Don't forget your pillow. If your car does not have driver-side lumbar support, put a small pillow behind the small of your back. When sleeping as a passenger, a pillow dampens the vibration, making it less likely that you will wake up with a headache.
Don't forget your drugs. Caffeine is fine if it's not a problem for you, but don't forget real drugs like headache, sinus, and upset stomach medicines, and keep them where you can reach them; night driving with a splitting headache is hell.
Don't get too cozy. Keep the ambient temperature a bit cool, though not cool enough to give you chills. As a last resort, drive with the window wide open (don't depend on this to get you further than the next exit, however.) Bring a blanket for the passengers so they stay warm.
Most important of all, don't obsess on making good time. Stop frequently whether you think you need to or not, and if staying awake becomes a struggle, give up! The best stops are places where you can get on your feet for awhile and get a little exercise. One thing we find helpful is walking laps at 24-hr Walmarts or Targets. (Just go around the perimeter of the store a couple of times; the walk will help re-energize you, and it's a safe, lighted place.)