Three Phases of Weight Pull Training
1) Drag Weight Training
Get a drag sled and some barbell plates. Chain the sled to your dog's harness and start with light weight (just the chain itself or maybe the empty sled) and pull this slowly with your dog, roughly 400m, three times, with 5 min water breaks in between. If you do three times a week you will see a great increase in your dog's confidence and form, which should increase the amount they pull on a cart or rail system as well. Each week see if you can add 5-10 more lbs to the sled, making sure your dog is exterting effort to pull the sled, but not quitting or getting frustrated, keeping in mind to continue to go SLOW, so your dog gets the idea of the right movements, and be sure to give huge praise for forward movements. You need to take baby steps, and make a big deal out of little wins.
2) Slat Mill / Carpet Mill / Hand walking
Another way to ensure your dog is building proper strength and wind is to exercise them often. A great and quick way to do this is through running them on a carpet mill or slat mill. You can get equal to a 60 min walk in about 5 mins (great for people with multiple dogs). If you don't have a slat mill or carpet mill, good ol hand walking on a leash does the trick too.
3) Cart/Rail Training
The third portion of pull training is to actually get your dog on a cart or rail system and pull, similar to what they'll see in competition. Depending on your training of thought, yo can do less reps of higher weight, or highers reps of lower weight. The aim would be to get 30 successful pulls of increasing weight, and increase the weight every few sessions.
Training Articles:
Mountain Gator Kennels (Mark/Eli Landers) - Weight Pull Training
Brown Dog Design - Weight Pull Training
Working K-9 Basic Intro to Weight Pulling
CDPits - Training for Weight Pull