Another approach (generally last resort) is to try to drill it out with a left hand bit (see micro tools). If you've been a good little boy or girl, Santa will allow the counter clock wise bit to back out the screw enough to grab and turn with pliers. If not so good, then you'll need to drill out the old screw and tap for a larger screw. Another alternative is to drill out the screw head and use glue in that spot instead of a screw (obviously you can't remove that part again).