First, cut pieces into the size you want. I promise not to judge you. Lay them flat on a cookie sheet and freeze for about an hour.
After an hour, the pieces will be pretty hard. Throw them in sandwich bags and put back in freezer. This way, the frosting doesn't all mush to the bag and stuff. When you want cake, just take out about an hour before you want to eat it and put cake on plate. It will be as good as fresh baked after it defrosts.