Energy is a funny thing for a person. Up to a certain point, the more a person uses, the more they tend to have. Contrary wise, failure to make use of our own energy tends to lesson it.
My point? This can be used to the advantage or disadvantage of the actor during a show.
Most people find the energy they need while ON stage. What about off stage? Just observing some videos of previous performances of mine, and looking back on my memories of them, it is easy to recall that many people drop their energy like a ton of bricks as soon as they are off stage. Sometimes, when there is a blackout, people can be seen dropping their energy even before they leave stage. I assume such people assume that since they are not acting, and cannot be seen, it becomes less important to keep a certain level of energy up.
I of course do not advise running around the outside of the theatre whenever one is off stage, in order to keep up energy. That would certainly pass a threshold, and merely fatigue an actor. Still, I believe that loafing about in the green room, lying on the floor, relaxing totally on a couch, or just tuning out in general when one has a break in a show of more than 5 minutes is fool hearty.
Keep a slow walk or pace going on around the building, so long as it does not bother anyone. Run your next scene silently at triple speed while you wait. One of my favorite energy maintenance tricks while off stage is shadow boxing. Whatever method you choose, be sure that your body and mind do not slip totally into "rest" mode. "Rest" mode is, for most people, only a step or two away from "non-performance" mode. Of course, no one wants to rush out on stage in that mode.
This goes for what we visualize and talk about as well. You may feel fatigue, but going about backstage telling anyone who will listen, "I am so tired! I am exhausted! I have no energy", is a self fulfilling prophecy that will only enhance your tiredness. Talk about how well the show is going, or what you are going to try in the next scene. That way even if physical fatigue has started to set in (and if you are working hard, it will), your can at least convince you mind you have some more to give before curtain call.
That is not to say that a person should not sit down and relax for a few minutes after a big musical solo. By all means take a drink of water, catch your breath, gear down a bit. Just resist the temptation, (and we all have it at some point) to leave our energy on the stage. Staying just a little limber and a bit warm when you are off stage will increase your chances of being red-hot when you get back out there.