Apply penetrating oil to the screw and let it soak
Tap the screw head lightly to help the oil penetrate
Use the correct screwdriver or bit with firm downward pressure
Try turning the screw slowly and steadily
Use a rubber band between the bit and the screw head for extra grip
Heat the screw briefly with a heat gun or soldering iron
Use locking pliers if the screw head is exposed
Cut a new slot in the head with a rotary tool if needed
Use a screw extractor on a drilled pilot hole
Drill the screw out if all else fails
Replace the screw after removal