Sheldon Brown wrote:
For steel seatposts stuck into steel frames, where the problem is often caused by rust, penetrating oil can do wonders. If you don't have real penetrating oil, any light lubricating oil is better than nothing, but penetrating oil is made specifically for this purpose, and you should buy a can if you don't already have one.
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. [Jobst Brandt doesn't think this works, because the ammonia won't penetrate -- see his comments on stuck handlebar stems. Drano drain cleaner in water also dissolves aluminum oxide. Leaving the frame upside down with the seatpost soaking in one of these liquids may possibly free the seatpost. With the frame upside down, you might also run liquid down from inside as described in additional suggestions.-- John Allen]
[Temperature-differential method, which is applicable to any seatpost material: Buy dry ice (solid carbon dioxide, which melts at -78.5° C (- 108° F). If you have access to laboratory supplies, you might also use liquid nitrogen, which is even colder, though its cooling effect is not as great because it boils, forming a shield of gas around itself. Ordinary water ice also might work. Remove the bottom-bracket parts, cork the top of the seatpost if it is open, and with the frame upside-down and a saddle attached to the seatpost, drop chips of (dry) ice or pour liquid nitrogen down the seat tube into the seatpost. Then hold the saddle down on the floor with your feet and twist the frame. You may also warm the seat tube by pouring hot water onto its outside. Wear winter gloves, boots and socks. Do not touch dry ice, the seatpost or other parts chilled by dry ice or liquid nitrogen. I thank John Newgard for this suggestion. Sheldon's original suggestion follows -- John Allen]
Drain cleaner isn't ideal as it will remove anodizing on the stem wedge, assuming your going to re-use it.