Methods for preparing metallocene-based catalyst compositions include the steps of contacting an alcohol compound and an organoaluminum compound for a first period of time to form a precontacted mixture, and contacting the precontacted mixture with an activator-support and a metallocene compound for a second period of time to form the catalyst composition. Such catalyst compositions can contain an activator-support, a metallocene compound, an organoaluminum compound, and a dialkyl aluminum alkoxide, and these catalyst compositions have increased catalytic activity for the polymerization of olefins.