Replace water with milk or buttermilk
Add an extra egg or two egg yolks
Use melted butter instead of oil
Add sour cream or Greek yogurt
Add instant pudding mix
Add vanilla extract or other flavor extracts
Add a pinch of salt
Add espresso powder to chocolate cake mix
Add citrus zest to vanilla or lemon mix
Fold in chocolate chips, nuts, or fruit
Brush layers with simple syrup after baking
Use whole milk instead of low-fat milk
Bake in a smaller pan for thicker layers
Do not overmix the batter
Do not overbake the cake
Frost with homemade frosting
Add a filling between layers
Toast coconut or nuts for topping
Add spices like cinnamon or nutmeg
Use coffee instead of water for chocolate cake
Add mayonnaise for extra moisture
Add cream cheese to the batter
Add a tablespoon of mayonnaise or mayonnaise-based substitute
Add a tablespoon of cornstarch for a finer crumb