This thread has been pretty active, so I thought I'd add an idea or two.
Here is a link for a video of a very effective and safe (for horse and handler) method to break a horse from pulling back. Archer isn't very eloquent, and the video is pretty low quality, but the method is effective. Any reasonably experienced horse owner can handle this safely.
Equipment required: A strong, UNBREAKABLE
halter, such as a well-made flat-braid nylon halter (leather halters and most of the 1/4" rope halters will break), a very strong and very long lead (Archer uses a 60' 1/2" rope that looks like an old lariat), and a very stout hitching post or rail, leather gloves.
Halter the horse. Take a long rope, and either tie it to the halter ring or run it through the halter ring, under the throat and tie it with a bowline hitch around the neck near the poll, like a neck rope. Present the horse at the hitchrail and run the lead around the rail one full turn. Hold the lead rope trailing end from a distance of 15-20 feet, so you are out of the way when the horse pulls (that's why you need a very long lead and gloves). Get the horse to pull, startle it if necessary. When it pulls back, keep pressure on the lead so that the lead is tight and puts pressure against the horse, but still allow him to move backwards as he pulls (which will keep him from lunging forward). As he stops pulling, pull on the lead and apply pressure until the horse is back up at the hitching rail again. Keep pressure on the lead until the horse gets back up to the rail, where he should be, then relax it. Keep doing this until the horse decides pulling back isn't getting him anywhere. Pretty quick he'll decide pulling isn't worth the effort. Safe, effective, and just a couple of sessions will normally do the trick.
I like this technique better than any other I've seen. I've seen a rubber inner tube used with success, using the same concept, but sometimes a horse will lunge back against the pressure of the tube, once it reaches the limit of its stretch.
One of the greatest survival instincts of a horse is preservation of energy. If a horse doesn't gain what it wants when it expends energy, it will eventually stop doing that action. This technique safely teaches a horse that he can't get away, and pulling gets him nowhere. It doesn't take long to fix the problem.
The same concept works for training a horse out of headshyness. Rub your hand up his face and continue right off his ears. By the time he jerks his head, your hand is already gone. Just keep doing that until he quits jerking. Pretty quickly, he learns that jerking his head just makes him tired and doesn't do any good.