Maybe I shouldn't post this here, but ...

My neighbors showed me a neat trick,  simply turn the post driver upside down and lean it up to the nubs on the post, while slightly pushing the post away from you, and then pull the post back towards you and the bottom edge of the post driver (upside down, so its now at the top) acts as the 'jack' against the nubs.  2 or 3 nubs and its loose enough to pull by hand. 

Disclaimer: It works a whole lot better when the ground is solid enough to keep the upside down driver from sinking but not so dry or frozen that the post won't budge.  In the tough conditions, mechanical means are needed.