I prefer to do it more often and keep things fresh. While there surely is some benefit to flushing the whole system, I haven't necessarily seen it.
I agree with Bill's concept for the most part, but mine is a bit more "expensive", foolproof and time-insensitive...
I drain the radiator (or if feeling especially lazy, just suction out the coolant in the pressurized expansion tank on cars that are set up that way). On some cars with an integrated expansion tank on the radiator (some BMWs for example), you can actually "suction" out all coolant through the expansion tank, out of the radiator.
Anyway, drain the coolant from the radiator, refill with 50/50 of the right chemistry. Close it up, and drive. Some time in the future, repeat.
I do this when I think of it and have time, and shoot for a few good turnovers every 36k or so. I generally do at least one exchange every year to keep things fresh.
Since Im feeding 50/50, there is never any worry of an improper mix - it is just right there, correct as required. A gallon of distilled and a gallon of concentrate provide you two gallons to use, and will likely last you 2 years.
I also run Schaeffer's clean and cool in my cars.