Apply penetrating oil and let it sit for 10–30 minutes
Use the correct screwdriver bit size and type (often a slightly larger or different type helps)
Press firmly while turning counterclockwise
Try a rubber band over the screw head and press the screwdriver through it
Use needle-nose pliers or locking pliers to grip the screw head and turn
Tap the screwdriver handle lightly with a hammer to help break the bond
Use an impact driver (if available) with a matching bit
Cut a new slot in the screw head with a Dremel or rotary tool, then use a flathead screwdriver
Drill a small pilot hole into the center of the screw, then drill progressively larger until the screw threads release
Use a screw extractor (easy-out) with the correct size and apply steady counterclockwise force
If the screw is accessible, heat the surrounding metal (carefully) and try turning again
For stubborn screws, weld/braze a nut or bolt onto the screw head, then turn the nut/bolt counterclockwise
If all else fails, remove the material around the screw (grinding/chiseling) to free it