Use the correct screwdriver bit and apply firm downward pressure
Try a rubber band between the screw and the driver for extra grip
Tap a slightly larger bit into the screw head with a hammer
Use a manual impact driver
Apply penetrating oil and let it sit
Grip the screw head with locking pliers if it is exposed
Cut a new slot across the head with a rotary tool and use a flathead screwdriver
Use a screw extractor kit
Drill a small pilot hole and use a left-hand drill bit
Drill out the screw head if necessary
Replace the screw after removal