I recently made a nipple torture device using clothes pins, a threaded rod, springs and a wooden cross member.

Using the wooden cross member I attach clothes pins via some small springs to each side. Then the threaded rod sits center to the chest and attach the clothes pins to the nipples. To make things gradually more painful you turn the wing nut so it pushes the wooden cross member away from the chest, pulling on the nipples. I also have included an upgradeable part, substitute the clothes pins with alligator clips.