Can you do anything to make power tubes last longer ?

What is "normal" is not necessarily ideal for tube electronics, particularly vintage gear that was built for lower voltages that were prevalent when they were made.  As I mentioned above, my local dealer who only sells tube amplification often has customers measure their outlet voltage and then orders the right transformer for them to use on the outlets supplying their amp and other tube gear.  He prefers to run the gear at much lower voltages than is common.  This is usually the cure for premature tube failure.  He also has his customers bias amps on the conservative side.  This is consistent with Atmasphere's recommendation.  

My normal voltage has been, for the longest time very steady at 117 volts at the outlet, but has recently crept up to 119-120.  My amp is cathode biased so I cannot do adjustment, but typically, cathode biasing means conservative levels.  If my voltage rises more than this, I too will consider a transformer.

If you’re talking about the US, you are mistaken @atmasphere. Nominal voltage in the US is 120VAC $$\pm 5\; percent,\; so\; anything\; between\; 114VAC\; and\; 126VAC\; is\; normal.\; That\; is\; established\; by\; ANSI\; C84.1\; standard.$$

@cleeds I stand corrected. 

However if the line Voltage is consistently 126V you'll find that tube life is curtailed with many tube amps and all if they are vintage. 

I recently worked on an Eico tube amp that was 'eating tubes'. The amp was built when '110V' was the norm. The output tubes were conducting way too hard. They were cathode biased so I was able to change the cathode resistors and calm things down. 

The only thing that can come to mind about making your power tubes last longer would be to set their bias a little under than the recommendation for the amplifier. This will not hurt the amp or the tubes in any way. The manufacturer has a recommended set point for where the power tubes should be biased to get the best sound and operation from the amplifier but by adjusting the bias to your liking, you might be happier. And by dropping it down a bit under the recommended set point the tubes with be running a bit cooler and should last a little longer. . Some people choose to run their tubes hotter which burn them out sooner and others run their cooler which makes them last longer. You’ll also hear a difference as you adjust the bias. I’ve experimented with this on my Canary 301 mk 2 300b amplifier yrs ago. 

In other words, I can set the bias as low as I want to until the point when I hear the deterioration of the sound quality. Setting bias by the ear not by manufacturer's recommendation.

The designer and builder of our monoblocks uses the same Dodd Balanced Power Supply to power his builds that we use to supply power to our system. They makes exactly 120 volts at 60 Hz. The 75 lb. laminated core transformer takes whatever comes from the wall outlet and makes a +60v and a -60v rail, both at 60Hz, and then joins them at the outputs. The amps are optimized for KT77 output tubes and can run no other type of output tube. I foolishly tried all of the usual suspects, even though I was told by the designer/builder that it wouldn’t work; he was right, can’t get any other output tube type to bias correctly. Had a lot of Octets for sale for a while :).

Anyhoo, I feel very lucky about the amps being designed and built based on the same power supply; apparently, pretty darn optimal. Accordingly, I’ll say again, for tube amps, it’s worth using a Balanced Power Supply (or, I think, a power regenerator) to assure that your amp(s) see 120v at 60Hz.