No more killing innocent animals in these so called "shelters." It would be like killing a baby in an orphanage. Instead, expand the shelters (make them bigger), and keep all the animals as long as you can. Not only that, but also give the animals more space to play, to interact, to live. They deserve it. And when the time comes to finally let them go, the least you can do is not kill them, but rather put them in a safe enough place, like maybe put a cat on the street or some kind of wild animal (if there is one, which I doubt) in the wild, where it can at least have a chance of surviving. Also, go door to door, offering people these animals. Who wouldn't want a free pet dog, if offered one? 

How could all this be ensured, I hear you ask? Well, simple. Make a law against kill shelters!