Almost every other superhero has a mask or a hood to hide their secret identity. But all Clark Kent uses is a pair of glasses. Is that enough to conceal the identity? What is the logic behind this?
this article explains brillantly why he uses just glasses http://witnessing101.hubpages.com/hub/Why-Supermans-Costume-Actually-Makes-Sense
of course there is the other side of the arguement that superman is the true persona Superman stands alone. Superman did not become Superman, Superman was born Superman. When Superman wakes up in the morning, he is Superman. His alter ego is Clark Kent. His outfit with the big red S is the blanket he was wrapped in as a baby when the Kents found him. Those are his clothes. What Kent wears, the glasses the business suit, that's the costume. That's the costume Superman wears to blend in with us. Clark Kent is how Superman views us. And what are the characteristics of Clark Kent? He's weak, unsure of himself... he's a coward
That is actually a trope in itself (Warning! Link to TVTropes, a known time sink.). There are many, many explanations (superior facial muscle control, costume distracting from the face, mild hypnotic abilities) given under the trope namer heading. Pick any one you like.