Apply penetrating oil and wait
Use the correct screwdriver or bit
Press down firmly while turning
Tap the screwdriver lightly to help it seat
Use a rubber band between the screw and driver
Try a manual impact driver
Heat the screw carefully if appropriate
Grip the screw head with locking pliers if exposed
Cut a new slot in the head with a rotary tool
Use a screw extractor
Drill a small pilot hole if needed
Reverse slowly and steadily
Stop if the head strips further