The main reason his secret identity works is that it's more psychological than it is physical. For most people, Superman is seen as a god. Someone whose only pupose is to serve and save. I think most people just believe that he's always watching and waiting, ready to help anyone and everyone at a moment's notice. People aren't going to think that this is someone who works 9-5, has relationships, or lives a normal life similar to theirs. (For example, in the recent "For Tomorrow" storyline, Father Daniel was surprised to find out that Superman sleeps.) It just doesn't dawn on people that Superman has a secret identity. Besides, look at Batman, Spider-Man, and others. They all wear masks, so people know that there's another person to go with that identity...they know that there's an actual person under that mask...this is something that Superman just doesn't have to deal with...he's pretty much hiding right out in the open and it works. And just look at Superman...he's this bright and dynamic force in the world...look at Clark...a mild mannered, average, and dull kind of everyman...the fact that the two personas are worlds apart make the fact that Superman is actually somebody else is an ideal that the average person in Metropolis just can't grasp.