Mashed potatoes are best when prepared and mashed right before you serve them. Keeping them in the refrigerator can ruin their texture, making them unappetizingly greasy and clumpy. If you know in advance that you will need to reheat your mashed potatoes, only mash the potatoes in advance -- add the butter, milk, sour cream and any other ingredients just before serving. If you find yourself unexpectedly saving mashed potatoes to warm later, however, you may still be able to create a tasty final product with some patience and extra effort.
Transfer the cold mashed potatoes to the warming container of your choice. You can warm the potatoes in several ways, including in a microwave-safe bowl in the microwave, in a saucepan on the stovetop, in a slow cooker or in the oven.
Add butter and milk to the mashed potatoes. The amount of each you should use depends entirely on your personal taste and the amount of mashed potatoes you want to reheat. The goal is simply to keep the potatoes from becoming too dry, so start with a little of each and add more if necessary during the warming process.
Reheat the mashed potatoes gently. Set a slow cooker to low, the oven to a low or medium temperature or the saucepan on the stove over medium-low heat. Cover the container with foil if you are reheating the potatoes in the oven. Stir the mashed potatoes regularly during the reheating process. If you are using the microwave, heat the potatoes on high, but stop the heating process regularly to stir them. Heat the potatoes until they are hot.
Add more butter and milk to the mashed potatoes if necessary to bring them back to your desired consistency. Serve and enjoy.