Unfortunately, a bond is formed when water reacts with a steel frame and an aluminum seatpost forming aluminum oxide. This bond makes removing the seatpost very difficult. One way to remove a frozen seatpost is to dissolve the aluminum oxide with ammonia.
1. Remove the crankset and bottom bracket.
2. Use a funnel and carefully pour ammonia into downtube and let it sit overnight to allow the ammonia to penetrate all the way through the downtube.
3. Flip the bike over, grab your saddle with both hands and twist. This is very difficult and took every bit of strength. Eventually, the seatpost will budge a very small amount.
Prevention is far more simple than this solution. If you are stuck, the Sheldon Brown site has a comprehensive article about removing a stuck seatpost.