Use the correct size socket or wrench first
Apply penetrating oil and let it soak
Tap the bolt head lightly to help the oil penetrate
Try a six-point socket instead of a twelve-point socket
Use a bolt extractor socket
Clamp locking pliers tightly onto the bolt head
Heat the surrounding metal carefully if appropriate
Strike the tool with a hammer to improve grip
Cut a new slot in the bolt head and use a flathead driver
Use an impact driver or impact wrench
Weld a nut onto the rounded bolt
Use a left-hand drill bit to drill into the bolt
Drill out the bolt and re-tap the threads if needed
Use a screw extractor if the bolt is drilled
Replace the bolt after removal