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