Apply penetrating oil and let it soak
Use the correct size socket or wrench
Tap the socket onto the bolt head with a hammer
Try a six-point socket instead of a twelve-point socket
Use locking pliers if part of the bolt head is exposed
Use a bolt extractor socket
Try a manual impact driver
Heat the surrounding metal carefully, then try again
Freeze the bolt with penetrating spray if appropriate
Cut a slot in the bolt head and use a flathead screwdriver
Drill a pilot hole and use a screw extractor
Drill the bolt head off if necessary
Remove the part and grip the remaining bolt with pliers
Weld a nut onto the stripped bolt if possible
Replace the bolt after removal