Use the correct screwdriver bit and press down firmly
Try a rubber band between the screw and the driver
Tap a slightly larger bit into the screw head
Use a manual impact driver
Apply penetrating oil and wait
Grip the screw head with locking pliers if it is exposed
Cut a new slot in the head with a rotary tool or hacksaw
Use a screw extractor
Drill a small pilot hole and extract the screw
Drill out the screw head if all else fails