But one that probably wasn't covered, when all the above, including the dry ice in a quart plastic bottle wrapped around the seat tube to allow the aluminum post to contract at a different rate then the steel frame, method doesn't work.
Rent/use a Sawzall type saw, steel blade, cutting off the seat post about 1 inch above the seat tube.
Then VERY CAREFULLY cut the inner wall of the post until you free the post, "taking care" to not cut into the seat tube.
Really. I had to do this once and it does work, but is the absolutely the last resort.
Steve B.