Anybody that knows Clark Kent would immediately realizes that he is Superman (Superman doesn't wear a mask, it's a huge 6"3' guy, the only difference is that he wears glasses when in 'Clark Kent mode', what is the explanation for nobody noticing that both men are alike?)
I remember reading, long ago, about this same thing. And the answer that was given by someone who worked on the comic (I'll try to find a link) is that it's a matter of perception. When you see Clark Kent, you see a nerdy, bumbling, clumsy, slouchy person. Yet when you see Superman, you see a tall, handsome, confident, strong man. In the minds of people who know Clark, how could he ever be Superman?
Essentially, his disguise is being everything Superman isn't. So subconsciously, you think there could never be any association.