I had this problem with the seatpost on my commuter. Since it's made of aluminum, my LBS fitter guy used some fine sandpaper to roughen it up a bit, the he put some kind of gel stuff on it. Hasn't budged since!

Of course, this is a BIG no-no with a carbon seatpost. The fitter said sometimes the post is just too smooth and it happens no matter how little you weigh.