Usually I would flip the bike upside down and spray penetrating oil down the fork so that it reaches the stem. Rinse and repeat every hour or two. The next day, I would flip the bike back over with the wheel in the fork, clamp the wheels with your leg and turn the stem to see if it's loosened up a little (it's easier with handlebars inside the stem). If that fails, keep trying that for the the next two or three days and if it still doesn't come out, then I resort to cutting the stem out.