@Tacomelt
I remember reading/being told that the reason why nobody recognizes superman and clark kent as the same person is because he emits waves or some shit that makes people see his face differently when he's Clark Kent, someone told me he went to a painter as superman and then as Clark kent and the painter had drawn 2 completely different people. I don't know if this is true though, i never bothered to check because i like the idea.
@Richman
There are almost as many explanations for the Clark Kent/Superman identity disparity as there are writers of the series. In addition to the "waves" thing, some writers have said its because he carries himself so differently between identities (like, Clark is actually an few inches shorter than Supes) and have different voices, that Kent is so unnoticeable that his features don't really stick in most people's memories, that people can't believe that Superman would ever bother being a normal human so don't even think of making the comparison, etc., etc.