extending life of drive system

Hi

I have just bought a new hybrid to cycle the 15 mile round trip to work.
What is the best way to get the maximum life out of the drive system.

On my last bike i used to run two chains and swap them every 300 (ish)
miles, degreasing and re oiling the removed one. I got a couple of years
(approx 7000 miles) before having to change the whole lot.

Does this sound like a reasonable way of doing it, or is it better to just
keep renewing the chain every ??? miles (suggestions appreciated).

Any advice appreciated

Gary