Last year I experimented freezing potatoes and I've been meaning to post the results. I tried two different systems and both have worked well:
1) Mash potato: I cooked and mashed as normal and then bagged cold mash potatoe, about 500gms per bag. To reheat I thaw and then heat the mash gently in a saucepan allowing the water to evaporate. If it then gets too dry I add either water or milk (I know, a bit silly allowing it to evaporate first but that seems to be the only way). The important thing is to mix it really well.
2) Roasting potatoes: Couldn't be easier. Peel the potatoes and parboil them. Then put them into bags and freeze. Roast them from frozen. The only thing you have to deal with is the fact that in the freezer they will stick together and so you have to separate them as soon as possible once they've started cooking. Also remember that the water on the outside of the frozen potatoes will make the hot (really hot) fat spit more so be careful.
I will be repeating this again this year - honestly, you can't tell the mash potato has been frozen and the roast potatoes are crunchier.