I've used the following method with great success, just don't get any on the paint or yourself.
Plug one end of the seat tube thoroughly. Fill it with a strong solution of water and lye. It will eat away at the aluminum and leave the steel untouched. Thoroughly rinse the seat tube out with cold water. Deactivate the lye with vinegar. Rinse with water. Usually takes a good day to free the post or stem, a couple days to completely eat it away if it's reeeally stuck.
Lye is dangerous stuff. Pm me if you have any questions.