Use the correct-size screwdriver and press down hard while turning slowly
Place a rubber band between the screwdriver and the screw head
Tap a slightly larger flathead screwdriver into the screw head and turn
Use locking pliers if part of the screw head is exposed
Apply penetrating oil and wait a few minutes
Use a manual impact driver
Cut a new slot in the screw head with a rotary tool or hacksaw
Use a screw extractor
Drill a small pilot hole and use a left-hand drill bit
Drill the screw head off if needed
Replace the screw after removal