Stuck seatpost (I've tried everything)

It's an old thread, but it's an old problem and it's all still relevant.

Adroit's post covers what you are trying to do.

Take the BB out, and cut the seat post close-ish to the frame.
Plug it with something that caustic soda won't dissolve, and fill the seat tube with a strong solution from the BB shell, with the frame upside down.
After a day or so drain it out, refill and keep repeating until there is no seat post left. The problem is that the alloy will react with the caustic soda and eventually it will cease to react. There isn't enough space in the seat post to get enough solution to dissolve the alloy, and the cheaper, thicker posts take more dissolving!