There is a story I read once about an ascetic who spent twenty years meditating until he was able to walk on water across a river. He told this to the Buddha, who wasn't impressed. "What a waste of energy," the Buddha said, "where for a penny you could have crossed the river on a ferry." The point of the story is that you don't gratuitously use your skills or powers for worthless things; you use them when the time is right.

In SW, there are two beings who are very powerful in the Force and knowledgable in its use yet seldom exhibit raw displays of that power: Yoda and Darth Sidious. Even though one is good and the other is as evil as it gets in this story, both exercise a restraint on using their own power. The first time moviegoers saw Yoda in TESB, he looked anything but a great Jedi Master. He was kooky, aged, and leaned on a cane. He didn't even seem to have a weapon. But when he used the Force to lift Luke's X-Wing out of the swamp waters, you knew there was more to him than met the eye. And even that didn't prepare moviegoers for Yoda's fight scene in AOTC 22 years later. He deflected Force lightning and scared off Darth "Count Dooku" Tyranus with his lightsaber skills, even though Dooku been a powerful Jedi himself. And immediately after Tyranus escaped, Yoda went back to his cane.

Sidious, who cheerfully exhulted in "power! Unlimited power!", wasn't exactly the sort to casually toss it around either. We only saw him use a lightsaber twice and use Force lightning three times, most of those times in ROTS when he fully revealed himself, and only when he was being attacked or challenged. Surely a malevolent being like Sidious would have enjoyed zapping people with his Force lightning for fun and shish-kebabing some rebellious Senators with his lightsaber. But even after he became Emperor, he didn't do that, even if such raw displays of power would have probably scared people even more into obeying him.

I think both Yoda and Sidious believe that one of the luxuries of power is that you can use it anytime and in any way that you want, but you can make that choice judiciously. Both also rely on a little bit of subterfuge, presenting themselves as elderly and physically weak, or as friendly politicians, because power can come from having others underestimate you.

Both would rather use their heads than to rely on supernatural powers. Yoda drew upon almost 900 years' worth of experience and knowledge while Sidious would rather manipulate and scheme to accomplish his goals.

Where they differ is of course, on their moral/philisophical outlook. It's that difference that distinguishes what Yoda believed power could accomplish from what Sidious believed. Yoda didn't use the Force to keep him from aging. When death came, he accepted it. Granted, he had learned from Qui-Gon the secrets of true immortality so he had no reason to fear physical death. I think Sidious really planned on living forever, because as a Sith, he probably believed there was either no existence after death or that any sort of afterlife did not afford the power of the Dark Side. (Even in the EU's Dark Empire, Palpie planned to live on in clones if he couldn't otherwise discover the secret of eternal life with Vader.) Ultimately, what true power meant for Sidious wasn't just the ability to electrocute Jedi Masters or rule an entire galaxy or even to turn the Chosen One to the Dark Side. It was the power to conquer physical death.