Can you do anything to make power tubes last longer ?

If the amp allows, does switching from Pentode to Triode make a difference?

Cryogenic treatment-surprised it hasn't been mentioned earlier.

Cryogenically Treated Tubes, the most cost-effective tweak for your audio system

 Of course, only for those who believe. 

I always keep a back up set of the full tube complement for each amp. I’m also checking bias every time I run the units --using a Fluke meter-- after the units have fully warmed up using a digital source for the first 45 minute to an hour. I do run them at exactly the recommended setting which is printed on the top plate, next to each tube socket). I often check them at the end of a listening session, too.

I used to never touch the tubes, using white cotton gloves to avoid any fingerprint oil, but abandoned that practice some years ago.

Lamm recommends replacing the driver tube--a 6N6P--every two years for optimal performance. I don’t use the amps constantly. I do know someone--perhaps the oldest Lamm dealer-- who runs his constantly. I get fairly long life from the amp tubes, which I buy directly from Lamm since the 6C33C not only requires burn in (which Lamm does, not sure what the failure rate is) but also has a peculiarity in the output stage*--each 6C33C has a different range in which they properly bias and Lamm matches, by serial number, a tube suitable to the particular amp for that purpose. I thus never tried to buy the tube on the open market. (The particulars for this are not disclosed by Lamm but in talking with another tube amp engineer, he seemed to understand this-- it might be interesting to get somebody else who uses the tube, like Ralph @atmasphere to explain the technical details). Two are in each mono amp- one as the output tube, the other used for voltage regulation. (* I gather that the tube used for the power supply doesn’t require the same degree of matching but since I get them all from the manufacturer of the amp, I am not sure).

One other thing I observed --the only tube I roll in the amps is the 12ax7 and have long preferred an old stock Tele ribbed plate (over the smooth plate). Many of the so-called NOS are, as you likely know, "pulls." I have been using truly never used old stock 12ax7s, a little more money but they seem to last much longer and the bias on the amps seems more stable.

Really, don’t overthink it. The lifespan will be determined primarily by the amp’s operational parameters (bias, voltage), and quality of the tube. Enjoy your tube amp. Vintage tubes tend to be very good, and long lived. But these days, most of what’s left there are used and will have unknown hours (many???) on them already. Russian power tubes have been very good for me, but they do fail occasionally, and yes "spectacular" failures (catastrophic short) can happen and take out a resistor. Replace those tubes every 2000 hours, at most.

Some of the amps with individual tube bias pots can be run colder or hotter, to get either (maybe) more tube life or more "class A" respectively.

Don’t throw a blanket over a tube amp, but don’t bother with useless tube coolers either. You’d only need a fan if you’re sticking it in a cabinet.

Don’t leave it on for a few hours just to save an on/off cycle. Just turn it on when you’re ready to listen. Unlike some SS amps, it should be at its best in 10 minutes. Any hifi amp you have today should be starting the tube heaters up well in advance of the high voltage, so it’s a safe start for the tubes. You would be hard pressed to find a tube amp anywhere that doesn’t have SOME mechanism for soft start.

On/off cycling the worst, in rush current at initial startups hard on tubes, rectifier tubes and/or soft start circuits, rectifier tubes may be sacrificial here.  Heat is the enemy of all components, my 845 SET amps, nearly 1000V at plates, cooling fans beneath amp to push heat out, 300B amps much less stressed by the heat. Quality tubes with good reputation for reliability.

Some amps just harder on tubes than others, some manufacturers really push with high plate and/or heater voltages. Do your research on amps, determine whether there is history of failures, this is number one with me. For some years I was having failures with a certain tube, replaced loosely spec'd resistor in heater circuit with much higher spec resistor, no further problems.