Apply penetrating oil and let it soak
Tap the bolt head lightly to help the oil penetrate
Use the correct-size wrench or socket
Turn slowly and steadily
Try tightening slightly first, then loosening
Use heat on the surrounding metal
Use an impact wrench or manual impact driver
Clean exposed threads with a wire brush
Add more penetrating oil during the process
Use a breaker bar for extra leverage
Freeze the bolt with a cold spray after heating the area
Cut a new slot or use locking pliers if the head is damaged
Drill out the bolt if it will not move
Replace the bolt after removal