I put a few drops of lighter fluid on the joint, let it soak for a few minutes, then slowly, gently pull it apart. It may need more than one application. Then I wash the model with soap and water. It works most of the time.

As to why I say do not light it...someone did a while back thinking it would speed up the process...yes he set the model on fire...no it did not recover...yes it was a 'doh' moment :(