Good advice above ^
Also, which harness do you use? As QuickSilver stated, the dog's chest should naturally be forced to turn back to you.
Jackson was never an awful puller but whne I had the harness on the back, he definitely did quite a bit of the sled dog pulling stuff. Ever since a day worth's of practice and using the Premier easy walk harness, he's been great. He's such a joy to walk, walks right beside or or behind me.
Here's a video of Jackson with the Easy Walk harness, took it with my cell phone, it was kind of hard to video and walk the dog at the same time, haha! but you get the gist.
yorkies :: VID00090.flv video by brit015 - Photobucket