Strategies for preserving tube life

I have a First Sound tube line stage with expensive (and glorious sounding ) Siemens Gold Pin 7308 tubes. I want them to last. My question is, what are the tradeoffs between wearing out the tubes by leaving the pre-amp on vs. stressing them out by powering up and down too often. So, for example, I power down when I leave town for two days. Should I also power down when I leave the house for 4 hours? Or when I go to bed at night? -Dan