What I did with my Max is I took him to a hallway in my apartment. I put the harness on him and kept in on for 15 mins. a day and then attached the leash. He of course was biting on the leash quite a bit. However during this time I had treats in my hand. I would give him one treat when the harness and leash was on just so he can associate this as a positive experience. I would take a treat and put it under his nose to smell. Then I would start walking in front with the treat just in front of his nose. When he got to the end of the hallway I would give him another treat and praise him. I repeated this a couple of times and a couple of days. He then started associating the leash and harness as a fun time. We then hit the outdoors where he had no problems. In fact he was trying to run ahead of me. He is now 6 months and he loves his walks.
I hope that helps.
The Dogs Obedience Training Blog