Investigate the reason for the creaking of the chair. Move it to another spot to try to stop the creaks. The creaking may actually be from the floor or from the chair hitting against another surface.
Locate the source of the creaking. Someone else should rock in the chair while you listen for the noise. This might be a wooden rod, like a dowel that helps support the rocker. Crossbars offer similar support and can sometimes creak as well.
Rub a product like oil soap or olive oil around the frame and joints of the chair. As an added bonus, your chair will smell nice and shine. If you use olive oil, you will go green as an advantage.
Jiggle these rods, dowels, supports and crossbars to see if they are loose, as the resulting friction will cause creaking. Insert wood glue, available at home repair or craft stores, into any loose spots. The glue will create a seal and stop the rubbing that creates creaks.
Test the chair to see if the creak is gone. If it is not, continue to oil and apply wood glue into any loose spots.