I've used a lot of different methods combined, and sometimes freezing the post with dry ice seemed to help it finally break free.
I had a stuck quill stem in my Pro Tour yesterday, and had already soaked it with PB Blaster from the bottom end and waited 24 hours, but still it wouldn't budge.
I decided to try a different approach, having used the propane torch on many of the bike's spoke nipples with much success.
So I blasted the quill with the torch for a few minutes, until smoke could be seen after the flame was removed for a moment.
The stem initially didn't budge, but after cooling for 5-6 minutes I tried again, and it cracked free with less force than I had applied before!!!
The stuck stem was holding me back from completing the refurb of the bike and using it on today's club ride in the hills, so I gave the torch a try and voila!
Highly recommended, though here one would obviously have to use more restraint with the heat, to spare the bike's paint!
