How can do you extend the life of your tubes?

Hello Warren,I would have a new set of tubes on hand .You can compare the
sound quality between the old and new to give you an idea how worn your tubes
are.Also to get an idea what is changing as the tubes age.I ve noticed in guitar
amps that the sound gets duller,less vibrant as they age .Kind of like a set of
guitar strings getting old and flat sounding.You can also keep a set of old tubes
to remind you how much better your amp is sounding with the new,or newer
tubes.Ive noticed some of the worn tubes seem to lose some volume as well.

Since we're only talking about non-power tubes, then neither of your questions should be a source of worry. Leave the tubes powered 24/7 and change them at fixed intervals. It could be every 6 months or 2-3 years. A very small number of vendors are selling truly unused NOS tubes, most are used, even when stated otherwise. If you're not hearing any distortion or gain imbalances, then don't worry about it.

Could someone please explain why it is thought to be a good idea to wait between turning a tube amp or preamp off and then back on again? How long should the waiting period be? Thanks.

02-16-13: Warrenh
how do you know your tubes are beginning to fail and your music is suffering from tube deteriration?

Generally, again reffering to light bulbs, tubes either are working or not working. You will hear failure as buzzing, becoming microphonic (hearing footfalls amplified), or some other audible clue. This is unlike cartridge wear, in my experiences. While a cartridge can slowly degrade over time, the tube's decline will be rather sudden. It will sound fine one day and then the next day you'll notice a buzz or hum being generated. Then you'll have to track it down to a specific tube.

Do you bring you little babies to the tube Doctor to be tested?

No, once an audible issue has been diagnosed in a tube, the doctor's equipment cannot save it, so why bother testing it?

Also, when one of my driver tubes goes, time to replace all of them?

You can, and many people do, but you don't have to. I would say it depends on the age of the tubes. If I've had the amp for 3-5 years, and one driver tube goes, I would probably replace them all. However, if the tubes are only a few months old, I would probably just replace the defective one. Again, think of light bulbs. Let's look at a chandelier, if one bulb goes, do you replace them all? I have a chandelier with 6 bulbs, recently 3 bulbs died within a 1 month span. I changed them one at a time. Should I have just changed all 6 at once, maybe. That's a personal issue that will depend on your own personal comfort level, OCD level and financial situation.

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.