If penetrating oil does not work, I remove the handlebar and in its place I clamp a 3 foot long one inch diameter steel bar. With a donor front wheel in the fork and the wheel between my knees, I try to twist the stem, being careful not to twist the fork.Sometimes this extra leverage is enough. If it is not enough, I swing the bike upside down in my repair stand. It is important to have a rigid repair stand. I have the Park FRS1 frame repair stand which they no longer make. It has the regular repair stand mounted to the top of it. With the bike upside down, I hit the bar next to the stem alternating sides each time I hit the bar. This technique usually removes very stubborn stems. A mechanic at a shop where I work recently told me that he could not remove a stem. At this shop, I do not have my special tools, so I loosened the stem bolt and hit the stem bolt knocking the stem further into the fork, that was all that was necessary to break it loose and remove the stem.