Physical exercises. Just tense and relax differents muscles of your body. You can shake your head, as if you wake up.
Changing the temperature. Reduce the degrees on the climatic installation, and yet it coud be better to open a car window, fresh air also works towards awakening.
You can wash up, splash cold water on your face or use wet wipe.
In order to avoid a sandy-gritty sensation, it is possible to instill eye drops. In general, any change may convince the organism for some time that it doesn't want to sleep.
Some people take off their shoes, some of them actively rub ears (there are many active points), someone makes light massage of eyes.
It should be mentioned once again that we are all different. Everyone has different fatigue threshold and for someone 300 mi is a feat, while professionals can overcome 900 mi at a time. You can choose a route in advance. Most of people prefer the direct bright roads, but some drivers quickly tire due to the route monotony and it is better to drive on the secondary routes, where the hiaghest concentration is required. But you shouldn't overestimate yourself - driving speed in this condition is lower and the total travel time increases. Everyone has his own method to delay the moment of falling asleep. Some people make their own refreshing drinks (as an example, cola with coffee), someone likes a tense driving and someone wakes up from erotic fantasies, other people wake themselves up with a slice of lemon or sour apples. But it is important not to miss the moment when all the methods were tried, but you still want to sleep and become fall on sleep at the wheel. You shouldn't push luck. Driving in such condition will not save your time, and may lead to serious consequences.