There's also the hugely annoying fact that all he has to do to disguise himself as a normal person is wear a pair of glasses....
If I remember rightly, didn't they also switch the parting and colouring of Christopher Reeve's hair for Clark Kent and Superman?
Cast twins for the roles of Clark Kent and Superman. One twin could get all muscled up to play Superman and the other emaciated to play Clark Kent. Just another power to be able to change his appearance somewhat. They would also play the characters differently being two separate persons.