I did once fall asleep at the wheel and I crashed my car in to someone elses. There were not any major injuries luckily but that was a very sobering experience to say the least. If things had gone just a little bit differently I could have killed myself or someone else.
I have tried many different things to keep myself awake since them from drinking coffee to singing loudly. And I have concluded that when you are tired there is really no way to wake yourself back up. The only responsible thing to do is to pull your car over and get some sleep.