Use milk instead of water. The instructions on the back of the cake mix will tell you how much water to add to the mix. To make it better, use the same amount of milk instead of water. For example if the mix calls for 1 cup of water, use 1 cups of milk. Use melted butter instead of oil. I have tried experimenting with the amounts of butter compared to the amount of oil called for on the box and have determined that the amount should stay the same. If the box asks for 1/2 cup of oil, use 1/2 cup of melted butter instead. Use TWO extra eggs. Especially in cupcakes that you plan to add filling. The eggs help the cake to be more dense and more able to hold up to the weight of the filling and the frosting.