Gillette and Man of Steel have put together a clever ad campaign with famous people postulating their theories of how Superman shaves (since our sun gives him the power of invulnerability, Earth razors wouldn't work on him).
There are videos on Gillette's youtube site by Bill Nye the Science Guy, Kevin Smith (actor, director+), Mayim Bialik (from Blossom and Big Bang Theory) and Mythbusters' Jamie & Adam. They're all pretty reasonable theories. You can vote on which you think is most probably. Here's one to get you started: