What is the best way to avoid falling asleep at the wheel?

I am way more afraid of driving when I am tired than intoxicated. I have found myself nodding off while driving a few times in my life and it is real scary. Fresh air, music, slapping yourself don't seem to work. So I do not think the answer is stimulation. Is the secret electrolytes, caffeine, hydration, adrenaline, sugar, adrenaline? Which drops and causes you to p out due to basic exhaustion?