This is something that might be hard to do, but when I changed my clay body, it made a WORLD of difference. I started using Kentucky Mudworks Dark Star. I make 12 inch plates all the time, and so far, I have had NONE of them warp or even crack. It has to be the clay, because when I used Buncombe White, I always had cracking or warping. And I have done nothing different. I do use bags of small pebbles like your rice bags. And I dry them slowly under plastic. I hope this helps.