For goodness sakes, don't mash them with a blender, food processor, or mixer. Just use a plain ole' potato masher by hand. The more you work with the potatoes, the more mushy and starchy they become.
Make mashed potatoes at the last minute. Potatoes don't like to wait for anyone - they give up their tastiness! If you must make them ahead of time, stick them in a crock pot on warm and fluff them before serving.