Encyclopedia of the American Pit Bull Terrier : HomePage
There's an article on weight pull training under the training pages. You can start the mental conditioning at your dog's age. Teaching them to go through the motions, that the thing dragging along behind them isn't scary, that it's sooooo much fun to pull, etc. But don't go heavy until later. I pulled my first dog in competition at 14 months, and I think I burned her out on it to where I ended up retiring her after she got her UWPCH. My current dog is 2.5 years old and I've been doing a lot more build-up with her with less emphasis on pushing her to the bitter end, and I already feel like I'm seeing a lot more potential out of her.