This method has allowed me to get out all of the stuck aluminum stems I have encountered:
1 - The ammonia trick seems to work, usually I flip the bike upside down and pour some through the bottom of the fork steerer and let it sit for a while.
2 - Put an old wheel in the fork and clamp a 25.4mm (1") bar in the stem. If you have an old set of weights the handle out of that should be heavy steel, long, and 1" in diameter. If you don't have that then an old steel mountain bike handlebar could work. Assuming whatever you clamp in the stem is long enough it will give a lot of leverage and you won't have to worry about messing up the wheel.
3 - Give the stem a couple taps down with a hammer, use a piece of wood to avoid damage.
4 - With the bike right side up put the wheel between your legs and grab the bar. Twist back and forth trying to break it free. Usually once it moves a little you should be able to keep going back and forth while pulling up a bit at the same time eventually working it out.
If it still won't come free try putting some liquid wrench or wd-40 in through the steerer bottom, let it sit upside down, then try again.