Smooth preview uses an algorithm to average over the existing vertices. Therefore there is no way to change how the smooth preview looks without changing how the base mesh looks. Instead of using the smooth preview, you could actually smooth it (mesh > smooth) and then tweak the vertices to get your desired shape. But a better workflow would be to change your topology to get rid of the triangles and route your edgeloops properly. As dave said, look in the topology sticky thread for some great examples. It's difficult to explain in words how to do this.