Many clients ask me if they should use a collar or a harness on their dog.  In reality, a harness is much safer if it is properly fitted.  Many dogs can escape from a poor fitting harness and easily be in danger when near a street.  

Many dogs will adjust quickly to a harness, however, I recommend you show it to your dog while offering him a few yummy treats.  Put it on and quickly reward him with affection and praise. Remove the harness and repeat a few times before you head out for your first walk.
 
Many reactive or highly territorial dogs do not enjoy the confinement of the gentle leader but do very well in a well fitting harness.

 
I certainly prefer a harness with a buckle in the front of the chest or if the buckle is on the dogs back, look for one with a martingale strap as shown below.  Notice the martingale strap on the back, as it tightens around Diamonds body much like a martingale collar.This concept is great for dogs that may try to back out of the harness.  It is also self punishing as it tightens when the dog pulls and loosens when the dog slows.
 
Of all the harnesses I have tried, I recommend the No Pull Freedom Harness, from Wiggles, Wags and Whiskers shown here on Diamond.  You will notice that it is completely adjustable, has felt covered straps under the dogs legs and a martingale to prevent your dog from escaping.
 
The leash is included and connects to front and back loops for greater control when needed.