I haven't tried it, but I saw an old dude where I used to live build a fence use a pretty elaborate method to prevent post rot. He dug the hole as usual but made the top 10-12" about 4" wider (making a counter-bore of sorts in the hole). Filled the around the post with concrete up to the step (he used wet but thats not the point) Then he slid an inverted plastic pot (3 gal?) with the top cut off over the post so the rim rested on the poured crete, then he filled the pot with more concrete. The next day he removed the pots and had a post set in crete with the top 8-10" shaped like a cone. Backfilled around the concrete cone with clear stone. The top of the concrete was just below grade and not visible. He said got the idea from how he makes his decks, but in that case the cone-part is entirely above grade.