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The worst suituation is when you're on a long trip alone. The possibility that a passenger will notice you dozing off is zero. The lack of passengers also eliminates switching drivers and the ability to keep yourself stimulated through conversation. I travel alone when I go see family a couple times a year (350 mi each way). I've have a couple of instances where I'd notice that the eyes are getting heavy. As with many interstates, particularly those with tolls, the next rest stop can be 30 miles or more. IMO pulling onto the shoulder for any reason other than a true emergency is a very, very bad idea--its dangerous and I'd hazard a guess, also illegal. I've found that talking (or even yelling) to/at one's self works quite well at staying focused and alert. Though I wouldn't recommend it in heavy traffic or at toll booths, else some concerned motorist may think you've jumped ship at the state mental ward and phone police.

As far as things to avoid... Drinking--this one should go without saying. It's a depressent on the CNS, hence even one drink serves no purpose but to make a bad suituation worse. Talking on the phone is best done at rest areas. Though, this seems to be more of an issue in short, commuting trips.