I have found that if you remove the "dropper" lid on the juice bottles, they have an expanded lip which "clicks" nicely into the hole in the top tank cap. Slide the cart back into the tank in the previously suggested manor, insert the empty juice bottle into the hole until it seats (mine "click" nicely into place), invert tank, and voila! empty tank! (well as empty as you can get them, anyway...) I have a clean, empty juice bottle which I use when I need to swap out carts or juices... I'm new to vaping, I have an OMG and just two tanks, but have been trying flavors left and right!
I put my name to everything I say and do. The Buck Stops Here.