Sheldon has a ton of solutions: 15 Ways to Unstick a Seatpost

Including:

Diagnosis
Before you get serious about a stuck seatpost, you should find why it is stuck. There are two possible reasons: The seatpost may be stuck mechanically, for instance by being the wrong size and having been forced in; or, the problem may be chemical, caused by corrosion.

# Aluminum seatposts frequently become stuck by corrosion also, and penetrating oil is almost useless against aluminum oxide. Fortunately, aluminum oxide can be dissolved like magic by using ammonia.

If nothing else works, the final resort is the old hacksaw blade trick.