Good sling curing practices

It's simple, a good egg-ish rock and some time. Make sure your rock is centered, and be patient.  It's wise to store your sling like this while your slinging in your dreams, because otherwise, it should be in your pocket or backpack ready for action.  And by "action" I mean, that rare moment when an open field, and a projectile and your sling collide in the same space-time.