How can do you extend the life of your tubes?

Ghosthouse,
My Hickok 539C does have provisions for a "Life Test". This setting reduces the filament voltage by 10%. In this setting the mutual conductance is not supposed to drop more than 20% compared to the "normal" setting.
I'm no expert here, and can't confirm the accuracy, but it sounds like this is what you're looking for in a tester.

Thanks, Mark. Helpful info. The other thing I was hoping to be able to do was match tubes so if I had a group of, say, EL34s of the same brand but with different histories, be able to put together a quad that could be managed by my amp's auto-bias circuit. I believe if tubes are too mis-matched auto-bias can't compensate...apparently not good for the amp. Is matching something a tube-tester can be used for?? Happy to be corrected on this issue.

Warren - apologies for hijacking but hopefully this is at least somewhat related to your question; that is, having a reliable piece of test gear would keep you from unnecessarily tossing tubes that still have some life in 'em (assuming no audible degradation in sound quality).

Hijack away...it's all good

Thanks Jmcgrogan2 & Elizabeth for the lessons and clarification. It is appreciated.

Here's an article that may shed some light on the subject of tube matching.

http://www.ramlabs-musicreference.com/virtues.html