I saw a mashed potato recipe demonstrated on TV for make ahead. The recipe is as follows:
8-10 potatoes peeled, boiled, and mashed
1 cup sour cream
8 oz. cream cheese
4 tbsp. butter
1/3 cups chopped chives
1/4 cups parmesan cheese
salt and pepper to taste

Add the sour cream, cream cheese, and butter to the hot potatoes and beat until smooth. Salt and pepper to taste, spread in a sprayed or oiled 9x13 baking dish, smooth, dot with butter, sprinkle the cheese and chives over the top. Cool, cover, and refridgerate for 2 to 3 days. To serve, bake in oven at 350 degrees until golden brown. Serves 8 to 10 people.

I have done a similar method to this, but used cheddar cheese instead of the cream cheese and parmesan cheese. I also added chopped green onions to the potatoes.