I, too, would advise you not to use a harness. Think about it, that's what is used with sled dogs to enable them to more efficiently pull. There are some "no pull" harnesses available and I've not used them so others on here may be better able to address those.
The three keys to properly training your puppy are motivation (yummy treats, enthusiasm from you), timing (making sure that any corrections you make are immediate) and consistency (every single time -- no exceptions!) Of those three, I find that timing is probably the most difficult to master because it is much more subtle, but no less important. Motivation and consistency are easier to pinpoint and determine if you are being successful.
Willow is a baby -- be patient, be interesting and enjoy this important time in his training. Good habits established now will reap huge benefits as he matures.
