Cool the Kiln
After the kiln has reached temperature, make sure all heat sources are off. Close any openings and leave the kiln to cool at its own rate. Generally, expect your kiln to cool for as long as it was heating (minus the overnight warm up). As a good rule of thumb, if you fire the kiln one day, let it cool overnight and unload it the next.
When you think the kiln has cooled enough, crack open the door. If any heat comes out, place a piece of paper in the opening. If it lights, the kiln is still too hot to open. If the paper does not light, but you hear pinging sounds, the kiln is still too hot to open. In either case, close the door immediately and allow the kiln to cool for several more hours. Make sure the pieces are completely cooled before removing them from the kiln.