Spit. Used to use commercial defog products till on a trip I tended to forget the stuff at the dive site. Started spitting, which I'd heard of. Haven't left that back at the room yet. Haven't noticed any detriment in effectiveness. While I've not done head-to-head comparisons vs. other options paying direct attention, hey, it works well. And if it works well, what's to improve on?
I generally spit twice, smear it around, later do it again, and have the mask on my forehead where it stays humid/moist inside & doesn't dry out. A little water let in during dive for rinse, and good-to-go.
Richard.