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, firm pressure to loosen it
Try tightening slightly first, then loosening
Use a breaker bar for extra leverage
Heat the surrounding metal with a torch if safe
Let it cool, then try again
Clean rust from the exposed threads with a wire brush
Use a bolt extractor or impact driver
Cut a slot in the bolt head and use a flathead screwdriver
Drill out the bolt if all else fails
Replace the bolt after removal