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Anyone has any experience with a suitable anti-fog spray or any substitute you've found useful (and relatively harmless!) for transparent, plastic mouthpieces? I think those type of mouthpieces, aside from plenty of other wonderful uses, makes a good pedagogical tool in terms of being able to see embouchure placement whilst playing, but the fog kinda gets in the way. Thanks in advance!

Lick it out with your tongue.

I've used an anti-fog fluid in a diving mask. Apply a small amount to the mask lens, then dip in the water. I don't know what it is, but looking it up, I see that divers often use saliva, and I suppose the point is simply to maintain a fluid coating on the inside surface.

Many moons ago, when I still had a motor bike, the recomended anti-fog for visors was a smear of washing up liquid. Can't honestly say I remember actually trying it.

The stuff I use in my dive mask is simethecone-based. It's a silicone polymer used in things like anti-gas/bloating meds. But, it seems like a thin film of food grade mineral oil might work, too. That shouldn't cause any problems downstream in the valves...