Those of you who have worked with me or poked around my website know that I am pretty consistent in recommending harnesses for dogs. Whereas harnesses used to be looked at as additional equipment to use if your dog couldn't walk on a flat collar, harnesses are now considered the new norm for leash walking by many trainers. I like harnesses because they prevent damage from being done to the dog's throat; many experts now are saying to avoid attaching the leash to equipment around dog's throats because they can damage the thyroid, esophagus, and trachea, and throw the dog's physical alignment off. Even if your dog walks politely on leash if they are startled they may pull forward, and it's natural human reaction to pull when we ourselves are startled, so it saves the dog from an accidental collar correction due to handler error.
This does not necessarily mean that all harnesses are created equal; some harnesses are designed for small dogs, some for larger dogs, and some simply aren't designed well at all. When I am looking for a harness for a dog the most important thing after correct sizing that I look for is that the straps of the harness sit far away from the dog's joints. If the straps sit on or too close to the dog's shoulders it can impede movement which at best will be uncomfortable for your dog and at worst could cause future injury and keep young dogs from developing properly. See the photo below of Phoebe in her balance harness; the straps are sitting far and away from the shoulders.
Note: There are some dogs out there who are so sensitive to wearing equipment that they will shut down completely when you put a harness on them; this can sometimes be modified with proper conditioning to the equipment, but if it cannot then this is one instance where I feel using a flat collar is appropriate.
If you have a particularly strong dog you want to make sure that the harness has both a front and back leash attachment, and may want to consider purchasing a double ended leash such as this one. Please remember, no piece of equipment (including prong, choke, and shock collars, which I do not recommend anyway) alone can teach your dog how to walk politely. If you don't teach the dog the skill first you will always be relying on that particular piece of equipment.
Recommended harnesses (the good!):
All of the harnesses listed below sit away from the dog's joints and properly fitted will not impede their movement. Most of these harnesses also have both a front and back attachment. If there's anything special to note about the harness I have listed it next to the harness name.
Balance harness: This is the harness you can see in the above and below pictures of Phoebe. The Balance Harness has multiple buckles and several adjustment points so if you have a mixed breed dog that is an atypical shape it's easy to fit this harness to them. It's also nice for dogs who are worried about a harness going over their head as there is an additional buckle for the neck of the harness. This is my new favorite harness!
Freedom harness: This is the harness I use with Regis. It has both a front and back clip attachment, but it has fewer buckles and adjustment points than the balance harness, so that's something to consider. See the picture below of Regis in his freedom harness. The freedom harness from the distributor will come with a double ended leash.