Apply penetrating oil to the bolt and let it soak
Clean off loose rust with a wire brush
Use the correct-size wrench or socket
Tap the bolt head lightly with a hammer
Try tightening slightly before loosening
Use steady, firm pressure to turn it
Heat the surrounding metal carefully
Use a breaker bar for extra leverage
Use an impact wrench if available
Reapply penetrating oil and wait longer if needed
Cut a new slot or use locking pliers if the head is damaged
Use a bolt extractor if the bolt is stripped
Drill out the bolt if it will not move
Replace the bolt after removal