Apply penetrating oil to the bolt and let it soak
Tap the bolt head lightly with a hammer
Use the correct-size wrench or socket
Try tightening slightly first, then loosening
Use a breaker bar for extra leverage
Heat the surrounding metal carefully
Cool the bolt quickly after heating
Clean rust and debris from the threads if accessible
Use a wire brush on exposed threads
Repeat penetrating oil and tapping if needed
Use an impact wrench or impact driver
Cut a new slot or use locking pliers if the head is damaged
Apply rust remover or rust dissolver
Replace the bolt if it will not budge