Use the correct screwdriver bit and press down firmly
Tap the screwdriver into the screw head to improve grip
Apply a rubber band between the bit and the screw head
Use locking pliers if any part of the screw head is exposed
Cut a new slot in the screw head with a rotary tool or hacksaw
Use a screw extractor kit
Drill a small pilot hole and use a left-hand drill bit
Apply penetrating oil and wait before retrying
Heat the screw briefly if thread locker is present
Drill out the screw head if all else fails
Remove the remaining screw shaft with pliers after the head is off