For those old enough to remember them (like TG

), the CRT is just a glorified thermionic valve.
As such, it will be losing emission from the cathode(s) from the first time it's switched on.
This will cause a (very gradual) display deterioration which manifests itself in poor focus and loss of contrast.
Obviously the best way to prolong its life is not to use it!
However, that rather defeats the purpose for which it was bought.
Provided there is not a catastrophic failure such as an inter-electrode short, keeping the contrast well down will eke out those precious electrons and should result in a long life.