I found this advice at "Bike Repair Tips from Kopp's Cycles":
5. Change chains. Most riders don't install new chains every 1,400 to 1,800 miles, or at least once a year. Ride long enough with an old chain, and it will wear your cassette and chainrings, so you have to shell out to replace everything, not just the chain.
I do not change chains by mileage, I use a gauge to check the "stretch" of the chain, and change at .075" .
Is this a good method, or should I incorporate the mileage method also?