In order;
Curse
larger head screwdriver GENTLY tapped in to the head of the screw with hammer
vise grips (if you have room)
Curse loudly
drill into the center of the head with a small bit, then again with one about half the diameter of the hole, screw into the hole you have made with a self tapping screw and attempt to unscrew the original screw, if this doesnt work, remove self tapping screw, carefully choose a bit just a bit smaller than the diameter of the original screw and drill out screw down the middle - at this point it will lose its structural rigidity and fall in on itself.
Curse loudly and colorfully (in a way that would make a sailor blush)
call up a mechanic (or a BMW, GM, or Fiat owner (they should have plenty of practice with tools and broken things

, bribe with beer and pizza to help you, usually will take a tap and die with reverse threads at this point
Good Luck!