Is it better to keep older appliances/cars longer to cut down on waste or to buy new energy efficient ones?

I have a 2003 Honda Civic that is fuel efficient and runs well. However, newer cars get even better gas mileage. We also have older appliances in our home that still work reasonably well, but would it be better to purchase new energy efficient models?
If you could provide websites that support your answer, that would be great.