Spray penetrating oil on the bolt and let it soak
Tap the bolt head lightly with a hammer
Use the correct-size wrench or socket
Apply steady pressure, then try tightening slightly before loosening
Use a breaker bar for extra leverage
Heat the surrounding metal carefully, then try again
Use an impact wrench if available
Clean rust from the exposed threads with a wire brush
Apply more penetrating oil and repeat
Use locking pliers if the bolt head is stripped
Cut a new slot in the bolt head if needed
Drill out the bolt as a last resort
Replace the bolt after removal