I found the ONLY non-drowsy way to travel... tried the other routes (coffee, dew, yelling, head out the window, slapping yourself, etc.).
The only tried and true method for killing the drowsy's is to carry a pack of sunflower seeds and a shell spit cup! If you don't already know how, learn to crack open a sunflower nut inside your mouth with only your teeth, spit out the shells and then chew the nut. You can put 10-15 sunflower nuts into your mouth each time... now you have to keep the rest of the nuts on one side while you try to open & spit the one your trying to eat. The task at hand WILL keep you awake!
I've became drowsy while only 2 hours into the dreaded 8 hour drive to visit THE MOTHER-IN-LAW.... I'd rather be driving............anywhere but there! The Sunflower method has allowed me to unfortunately make the entire trip without ever getting those dam drosey's again!