Can you do anything to make power tubes last longer ?

I use a variable speed cabinet fan at the lowest speed to pull heat out of my ARC preamp. Can't hear it and it certainly does not hurt. Of course, no data to say the tube life is being extended.

But, I only use the system an hour here and there during the week. 

Oh no, I never leave it on when I am not in the house, or overnight, for that matter.

Though my VAC has some circuit that shuts it down if power tube runs away.

I suspect you already had it. Don't leave on and less cycles. My  understanding is that tubes become less reliable... as in more likely to blow as well as have the sound quality slowly deteriorate. This is enough for me to change at recommended intervals. Three thousand hours takes a while even at the 2 - 3 hours a day, which I use mine. 

I wouldn't replace expensive and hard to get tubes unless they do something audibly/visibly wrong. But if you use new productions tubes, yeah, why not ?

This may not apply to your setup, but I have individual bias pots for each output tube. There’s a recommended bias range for each tube type . Running the bias toward the lower end of the bias range can supposedly get more life from them. It’s possible that the tube that runs at the lowest bias setting may last longer in general, but don’t know for sure. There’s no guarantee you’ll have optimal SQ with all bias ranges either, but that’s always subjective anyway.