Two methods I have heard:
drape a thin cloth into the carboy, keeping the four corners of the cloth "out". Then manipulate the carboy so the the stopper is near the neck. Then pull the cloth corners... kind of lassoing the stopper and pull.
The other person made a tool, kind of like a toggle bolt. With wire he made a loop, small enough to fit through the hole of the stopper and then with the loop thorugh the center of the stopper, placed a pice of wire to prevent the loop from pulling back thorugh, and pulled the stopper out.
The other method I have heard is a hammer...