Combine the cake mix with one can of soda and stir until blended. Leave out the butter/oil/eggs if the mix calls for it. Mix ONLY the cake mix and can of soda. Cook 10 minutes less than indicated time on box.
something easier, dear cake-lovers. Swap all but the cake mix for a bottle of soda. Stir 12 ounces of a carbonated beverage into your prepackaged cake flour, plop the mixture into a pan and bake it in the oven as directed on the box.