We take it off after the joint has cooked, and then cook again on its own.
Actually, to be more accurate - we sneak it off the joint before MIL chucks it in the bin (it's unhealthy, so she doesn't like people eating it) Take it home with us, cook it until it's all crispy and then eat it as a midnight snack.
I think rubbing salt into it helps with the crispyness though.
It has to be good to start with - if there isn't a decent layer of fat under the skin it won't crackle no matter what you do. But basically what Nancy said. If it's wrapped up, it's worth unwrapping in advance to give it a chance to dry out.