Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split.
DIY Concrete Leaf Bird Bath DIY Concrete Leaf Bird Bath - love this idea! Those leaves look like rhubarb leaves to me. Hmmm may have to do something like this. Especially if I do a cob house some day, this would be a great wall feature/shelf.