I have a couple of gmail addresses. One is used exclusively for work - emailing my boss and some colleagues - the other has been used to sign up on some sites I trust and is occasionally used by a couple of my friends. I have never changed the settings on either account, and do not publicise them. I have never had any spam emails and, with Firefox and Adblock+, I never get to see any ads either.
I suspect that it is a combination of not making either of the addresses public (by putting them out willy nilly on the net) and using unusual names which are not in the dictionary which keeps them spam free.