Other than assuring there is good drainage and that you plant them above the surrounding soil, the most you could also do would be to plant sasanquas and oleiferas as these have better resistance to root rot. Or you could buy a japonica that has been grafted on a sasanqua/oleifera root stock. Die back, though, is caused by a fungus that enters the plant through an injury so you can apply fungicides at times when there is some injury or when the plant looses lots of leaves (May-June usually).