1. A computer-implemented method, comprising: receiving publisher selections of advertisements from a plurality of advertisements, the publisher selections identifying selected advertisements for each advertisement slot in a document; associating the selected advertisements with an advertisement slot in the document; generating advertisement request code for each advertisement slot for inclusion in the document, the advertisement request code configured to issue an advertisement request for selection of an advertisement from the selected advertisements for presentation in each advertisement slot; providing the selected advertisements in response to advertisement requests; evaluating performance metrics for the selected advertisements presented in an advertisement environment; and adjusting, by a processor, the selection of selected advertisements that are provided in response to the advertisement requests based on the performance metrics and an override criterion, wherein adjusting the selection of selected advertisements that are provided in response to the advertisement requests based on the performance metrics comprises: substituting an unselected advertisement for a selected advertisement if the selected advertisement does not meet a performance threshold.