To prevent your layer cake from slipping, take a few key steps. First, make sure your cakes are completely cooled before you frost them, and level them to ensure they are even. Place a cake board or cardboard round under the bottom layer for stability. Apply a thin layer of frosting between layers to act as a "glue." Chill the cake briefly in the fridge after adding each layer, then apply a crumb coat and refrigerate again. Finally, frost the cake evenly and refrigerate until ready to serve.