At the risk of sounding like the anti-Christ here... I don't really care if Sheldon Brown's site recommends ammonia, because most alloy seat post to steel frame bonding/sticking isn't as the result of oxidation so much as lack of lubrication, small amounts of dirt/grit in the interface and subsequent spooling up of the alloy post when it's "twisted" in and out of the frame.
Hint here... do NOT twist your seat post in and out!
We've removed a couple of hundred seat posts from frames (this is a remarkably common thing and shops all over the country send frames to us for this... learn to lubricate, people!
) and the ammonia has never once worked. It needs to be pressed out... STRAIGHT out (remember... no twisting) and in 90% of cases your seat post will be toast so factor in the cost of new one.
Cheers