Measuring your ingredients without precision can cause a recipe to go drastically wrong. I always prefer to weigh my ingredients with a scale. The American "Cup" measurements can cause serious discrepancies.
Boxed cake mix asks for the most basic cooking ingredient ever, water. replace the water using milk or buttermilk in a cake mix increases the flavor and fat in the cake resulting in a richer flavored cake crumb.
Do not add prepared pudding, you want to add the dry pudding mix. For chocolate cake use a chocolate pudding mix and for white or butter cake use vanilla pudding mix. Use the small box of pudding 3.7oz.
This helps increase the flavor and create a softer cake.
Try adding a half cup of sour cream to your cake batter for an incredibly rich and extra moist cake. Even adding a couple of tablespoons of mayonnaise is wonderful. You may be thinking mayonnaise goes with something like egg salad but if you think about it mayonnaise is actually made of mostly egg yolks emulsified!