I had an aluminum seatpost stuck in a newer steel frame.
WD 40 didn't work.
PB Blaster didn't work. Letting it soak didn't work.
A home-made hammer rig had potential but didn't work.
Dry ice didn't work. The theory was that aluminum expands/contracts more than steel, so cooling it should loosen it somewhat. Maybe so, but not enough to matter.
What did work is carefully sawing it out. I sawed the post lengthwise using a hacksaw blade, being careful not to cut the frame. Took about 4 hours.
So, the advice above is all good and fine, but no guarantees that what worked for anyone else will work for you!