Use the correct size socket or wrench
Apply penetrating oil and wait
Tap the bolt head with a hammer
Use locking pliers if the head is exposed
Try a rubber band between the tool and bolt head
Use a larger socket hammered onto the head
Use a bolt extractor socket
Cut a slot in the head and use a flathead screwdriver
Apply heat to the surrounding metal
Use an impact driver
Drill a pilot hole and use a screw extractor
Drill the bolt head off if necessary
Replace the bolt after removal