Spray penetrating oil on the bolt and let it soak
Use the correct size socket or wrench
Tap the socket onto the bolt with a hammer
Try a six-point socket instead of a twelve-point socket
Use a bolt extractor socket
Clamp locking pliers onto the exposed bolt head
Apply heat to the surrounding metal if safe to do so
Strike the bolt head lightly to help break corrosion
Use a manual impact driver
Cut a new slot in the bolt head and use a flathead screwdriver
Weld a nut onto the stripped bolt head
Drill a pilot hole and use a screw extractor
Drill the bolt out completely if extraction fails
Replace the bolt after removal