Ensure that your butter and eggs are at room temperature for a better emulsion, leading to a lighter cake. Spoon and level your flour instead of scooping it directly out of the bag to avoid compacting, which can lead to a dense cake. Once you add the flour, only mix until it is combined. Overmixing can lead to a tough texture. Feel free to mix up the berries, but if you are using very juicy berries, you should slightly reduce the amount of milk to maintain the consistency of the dough. Always ensure that your cake layers are completely cool before frosting them to avoid melting and slipping.