My first 10 years I never broke a bow, badly bent, yes but no breaks. It wasn't until I became more proficient that they started breaking.
Like said above wood choice, proper design for the wood and bow style and IMO the most important, patience. I think lack of patience is the greatest killer of newby's bows.
A good way to keep from loosing too much weight while tillering is to draw to the ultimate weight while tillering. And don't forget to exercise after each wood removal session.