I work on expander type stems all day long (I work on Flying Pigeons), and this is a regular problem.

I haven't discovered any magic tricks - but occasionally the following works (make sure you've put a light-weight grease on the quill an expander to prevent them from binding together later on):

-Loosen up the stem bolt so the expander drops out of the quill (may need a brass hammer tap on bolt to loosen expander)
-Rotate bolt around while simultaneously pulling bolt slightly upwards
-Stop twisting when you feel bolt resist twisting (the expander has a notch in it you are trying to catch between the two halves of your quill)
-Begin tightening stem bolt
-As tension in stem bolt grows, place front wheel between your knees. Left hand on the bars, right hand with wrench placed on stem bolt, right thumb hooked over bars. Slowly and firmly tighten the bolt. Tighten in smaller increments when the bolt starts to feel firmly in place.

There will be a point when you try to twist the bars out of place and they will feel locked in place securely (or, rather, as securely as any set of bars can be). Test ride the bike around the block slowly. Try braking and turning, braking and track standing, at low speed with your weight on the bars. Try the braking and turning while at speed.

If all is good, then your stem is in place (you did make sure the height and vertical alignment wre right, right?!). If not, loosen and try again.