Drop peeled potatoes into a saucepan, adding just enough water to cover. Bring to a boil over high heat, and cover once boiling starts, reducing the heat to a simmer until the potatoes are tender when poked with a fork. Strain the water from the potatoes, and then whip them with an electric mixer. Add just enough milk to the potatoes as you whip, to form a smooth mash. Add butter, if you wish.

Perishable foods only remain safe to eat at room temperature for a short time. After making the mashed potatoes, store them in the refrigerator until you are ready to eat them. Scoop the mashed potatoes into a dish or container, cover and refrigerate. If you do not plan to eat the mashed potatoes for a few days, transfer to an airtight, plastic container.

When it's time to reheat the mashed potatoes, warm them in the microwave. Set the microwave to medium-high and reheat for approximately 14 minutes, stopping the microwave halfway through to give the mashed potatoes a stir. You can also reheat the mashed potatoes in a casserole dish in the oven. Warm the mashed potatoes in an oven set to 350 degrees Fahrenheit until heated all the way through. Fluff the mashed potatoes with a fork after reheating.

Mashed potatoes keep for three to four days in the refrigerator. After that, you should throw them out. During those three days, you can also freeze them. Frozen mashed potatoes keep for one month in plastic freezer bags. Thaw the mashed potatoes in the refrigerator, transferring from freezer to fridge 24 hours before you plan to eat them.