I gave a way a lot of cake when I was learning! Cake dummies are alright, but I preferred to practice on real cake for a couple of reasons. First of all, they don't really behave the same as real cake. They are lighter, so they can move around on you which is irritating, but they are also firm enough that you can get used to doing things that you will never get away with on real cake. In my mind, it didn't make sense to practice on something that would have me forming habits I would have to change later. Secondly, by using real cake I got to experiment with cake recipes right along with decorating practice. And by giving the cakes away I got a lot of feedback on what recipes people liked the best.