One good rule of thumb is to have the tires rotated at every oil change. With oil changes running about 3,000-5,000 miles apart, this is a pretty good interval.
The driver should also ask the mechanic to inspect the tires for wear and nails, and to check the valve stems for leaks. This is also a good time to have the tires balanced if the car seems to be handling in a way that's not quite right. It's much easier to inspect tires for wear and damage when they're off the car, as they will be when they are rotated.
Also have the mechanic make a note in your file about the rotation pattern, so he or she follows that pattern every time, to ensure even wear on the tires.