Make sure your ingredients are all at room temperature when you mix – eggs, butter, milk, and all. Cold ingredients will prevent the batter from properly aerating during the mixing stage.
Test your leavener to make sure it’s still active. Test baking soda by adding vinegar and baking powder by adding water. If the mixture fizzes, the leavener is still good. If not, buy a fresh new can/box.
If you are doing anything that involves folding with the batter, careful not to overmix.