Heat

• Metal is easiest to manipulate when it is heated, and that goes for a metal screw as well. If you have a threaded bolt or screw that is seated in a metal surface, try a applying direct heat with a torch to heat the screw up and make it easier to move. If there is any wood around then the torch is obviously a bad idea, so use a hair dryer to apply heat directly to the screw and see if that helps loosen the screw up. Do not touch the screw after applying heat to it; even with a hair dryer, be careful not to allow wood to catch fire.
  
  

Leverage

• If the head of the screw is still intact and you're able to easily place a screwdriver in the screw head, then you can use leverage to try and loosen the screw to make it easier to remove. Loosening a screw prior to removing it is important because a tight screw may break off and become even more difficult to remove; a very tight screw could create friction and cause a fire as you are removing it from a wood surface. Place the screwdriver in the screw head; use a wrench to turn the screw slowly. The added leverage of the wrench should make the screw move. Make sure your are applying sufficient downward pressure on the screwdriver as you are turning it with the wrench. A screwdriver with a square shaft works best. Try gently tapping the screwdriver with a hammer before turning the screw as well, just to loosen it up.
  
  

Paint

• Paint can seal the head of the screw as well as settle down into the screw hole and really make moving the screw difficult. Before trying to loosen the screw with a screwdriver, remove the paint from the head to give yourself complete access to the screwdriver guides in the head. Pour a little paint thinner over the screw and wait about an hour before trying to move it. The thinner should break up any paint inside the screw hole and make moving the screw much easier.