Instead of putting oil, eggs, water, etc. in your storebought cake mix, use one 12-oz. can of diet soda!!!
Use dark sodas (Coke, Pepsi, Dr. Pepper) for chocolate cakes and light (Mt. Dew, Sprite, 7-up) for white or yellow cakes.
I made one today and used a sugar-free cake mix and a diet Pepsi. It's 120 calories per 1/6th of the cake!