I (or my husband) usually just shake into a small bowl where we mix up spices. Since it isn't over heat with hot moist air coming up into it, we don't get much clumping in ours unless it is well over a year old.
But I have been told that they use small amounts of dry rice in salt shakers at restaurants exactly for that reason, to stop clumping. I would bet it would work well for other spices as long as the holes it shakes out of won't let the rice come out when you don't want it to.
I'm not sure if it needs replacing over time or not; I'm sure restaurants don't worry because enough salt gets used quickly enough.
The dry rice absorbs the extra moisture in the container as that is usually what causes clumping.
And I'm 100% garlic. Not a fan of onion powder, though actual onions are good.
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