leave upside down and drizzled in oil.
For those who can play the waiting game try 3in1 for a day or so.
For those who are impatient, use penetrating sprays and such like.
A bit of heat can help. Leave the bars in. If its a front loading stem, use good quality bolts to secure the bars.
Be aware that you can bend the forks if using too much force so dont use too big a bar length over standard handlebars.