Lie on your back. Using a parachute stretcher is best, but you can do it by just tying a rope so it pulls more on the balls than on your sack. Suspend yourself by the balls so your ass us up off the surface by quite a ways. See how long you can hold yourself up like that. Of course, it will be difficult to release with your balls stretched way out.
I use a broom stick across two chairs to hang from. Use your imagination but make it stretch real good.