1. A method for serving advertisements for a document having at least one ad spot of a first ad spot type and at least one ad spot of a second ad spot type, the method comprising: a) accepting an advertisement having a score and being provisionally associated with an ad spot of the first ad spot type; b) determining whether to move the advertisement from an ad spot of the first ad spot type to an ad spot of the second ad spot type based on the score of the advertisement; and c) if it is determined to move the advertisement from the ad spot of the first ad spot type to the ad spot of the second ad spot type, then assigning the advertisement to the ad spot of the second ad spot type, otherwise assigning the advertisement to an ad spot of the first ad spot type.