Powerful metaphors do two things: one, shine a light on something non-obvious about the target object or being (the "referent"), and two, hook this insight into what's already familiar, so you remember better.

When a calculus teacher compares derivatives to jumping out of an airplane and integrals to trying to jump back into the same airplane in the sky, this reveals that there's something massively unsymmetrical about the two opposite processes, something far from obvious at first sight. And since everyone's got images of jumping out of airplanes ready from watching television, yet few if any of the reverse (a somewhat shocking image), that's a very solid memory hook. It's the kind of metaphor someone can say once and people in the audience will have no trouble remembering for the rest of their lives. (It happened to me: my differential equations professor shared this metaphor one morning in 2006 and I'll never forget it.)

Something non-obvious about metaphors is that a good way to practice them comes from realizing that basically any two things can be metaphors for each other. There isn't any unattainable pixie dust that renders some things "metaphorable" with other things. You can take any two nouns or activities, compare them, and find links to mine for a metaphor. The dormant aromatherapy diffuser over there and a winking grizzly bear could be compared - it just takes some setup to highlight a thread of commonality (for example they both hibernate, and have steamy breath). The endless parade of answers to "Why is a raven like a writing desk?" is not really a function of a special connection between the two, but of the possibility of connecting any two things.

That knowledge - initiation into the metaphorical mystery cult through a brief demystification - helps you find the powerful ones. It's a curious case of lowering the bar to raise the bar. That's often true of creativity, but to say it's always true of creativity would be a dangerous oversimplification!