Use the correct screwdriver bit and apply firm downward pressure
Tap the screwdriver into the screw head to improve grip
Place a wide rubber band between the bit and the screw head
Try a slightly larger flathead screwdriver if the head is stripped
Use locking pliers if part of the screw head is exposed
Apply penetrating oil and wait a few minutes
Use a screw extractor kit
Drill a small pilot hole into the screw head for the extractor
Drill the screw head off if extraction fails
Remove the remaining screw shaft with pliers after the head is off
Heat the surrounding area carefully if thread locker is present
Replace the screw with a new one after removal