If you have to drive for a considerable time ALONE while fatigued, what's been described above as the nodding-donkey syndrome can sometimes be alleviated by favourite radio programmes or CDs. For example, I was once revived almost instantly by an album of Monty Python sketches I hadn't heard in years. Problem is: if you're doing it regularly, you may run out of stuff you haven't heard recently...
Then you must obviously stop at the first available safe parking area (note the previous post), preferably a motorway service area. I subscribe to the theory that a 20-minute nap is the best tactic, as in a multi-crew cockpit, because any longer will lead to you waking up feeling very uncomfortable and too sleepy to drive competently. However, you may be tempted to keep pressing the "snooze" button, which is not a good idea.
The trick to waking up refreshed after a short nap may lie in what I heard recently, but haven't tried out. As soon as you are safely parked, obtain and drink a fairly strong (by your own standards) coffee or other caffeine drink. Then set your alarm and crash out to sleep immediately. The caffeine will not affect your ability to go to sleep, because it takes time to kick-in, but it will certainly help to wake you up after about 20 minutes. Let me know if it works!