I think I know what you mean oldprof,
I dont' know what the technical name for it is, but I've alway heard it called "Road Hypnosis"... basically, if you just sit and stare at the road, the lines and reflectors going by in the same rythm will 'hypnotise' you and put you to sleep. One of the biggest things, as others have mentioned, is you keep your eyes and mind active.
Keep your eyes busy, look ahead, glance around, conciously be aware of what's happening around you. If you just sit and stare at the same spot, you're going to put yourself out! I'm a late night talk show person myself... I like to listen to good info, even some of the gospel/preaching shows, that kind of stuff, anything to make ya think... books on tape, whatever...and I talk a lot on the CB...(if there's not too many idiots.. LOL)
When there's no talk shows, crank up the Rock... no slow country, jazz or stuff that makes ya relax... something that gets the blood pumpin!
Other than that, just heavy on the caffeine til about 3 hours before sleepy time!
Dreamer