I clean mine every morning with my damp towel from the shower. Mine needs cleaned offer due to work. And, I clean it and the case with Zagg Foam twice a week to keep it sanitized.
My wife's 4s gets Zagg Foam everyday after she gets off work, she works at a hospital and is a trauma/infectious disease certified RN. She is around nasty stuff all day and so is her phone.
Sent from my iPhone 4S.