Patience, heat, PB Blaster, pressure, tapping and tightening before loosening all work. I have removed the screw tip from the driver and inserted it into the problem screw and tapped it a few times after letting PB do its thing. If all else fails drill the screw out and try removing with a screw extractor. They are available in a variety of sizes from your local hardware store, home center or auto parts supplier. Best of luck with your problem screws.
