Keep stroking. Your goal is to reach the edge while upping your pain tolerance. Go ahead and place a second clothespin on each ball. Try to keep stroking as you do this.
Now with your free hand I want you to twist each nipple clamp 90 degrees, hold for 3 seconds, and then let go. Alternate twisting each nipple as you continue to stroke slowly towards the edge. If you feel yourself reaching the edge already, then slow your strokes down even further. Take your time.