After some research I decided to bed my 6 X 6 deck posts in a product from GRA Services called Secure Set. It's a high-density expanding foam, and they use it out west to install utility and power poles.
Now bear in mind I'm building a very LOW deck--just 13 inches off the ground-- and I'm setting the posts 13 inches deep in this foam. I don't think I'd use it on a deck that's higher off the ground, but with an inch of post buried for every inch above ground at this low height, I trust it to help me maintain a rigid box structure in the deck framing.
I'm insetting and through-bolting 2 X 6 beams on each side of the posts, and the posts are spaced just five feet apart. On this, I'm setting 2 X 6 joists at 12 inches on center. I need this spacing to avoid sags in my Trex-like composite decking. I'm putting 2 X 6 blocking between the joists.
The project is almost done, and I swear I could park a car on this deck.
AGAIN - I endorse this method ONLY for very low decks. I live in Georgia and don't really have a frost line problem
Here's more on Secure Set.