You have probably already heard advice (or seen the ads) to use a harness to keep your dog from pulling. In truth a harness WILL NOT stop them from pulling, but it WILL give you more control over their body when they do pull, which can save you both. Also, when your dog pulls hard while leashed to a neck collar, it can seriously injure their esophagus, and a body harness solves that concern. Riding in your car, if you tether to a neck collar then a sharp stop could kill them, while a body harness acts more like a cross-body seat belt. To learn how to actually reduce pulling on leash, talk to a private trainer.
I recommend looking at these harnesses first:
Baydog Chesapeake
Blue-9 Balance
Ruffwear Front Range
Pet Safe 3-in-1
All four of these have a similar strap arrangement around the chest and shoulders, which you can see in the photos below. Some other harnesses have a strap that goes across the front of the chest and legs, and that strap can put harmful strain on the legs and shoulders during exercise, potentially causing arthritis type symptoms over time. The ones I recommend do not have that problem, instead distributing pressure evenly around the torso. Additionally they have connector rings at both front and back, so you can leash to both points, for dogs that need a little extra help. Think of the back point as an anchor and the front as a way to steer them in a safe direction. You can use two leashes, or a double-ended leash, and you can make any standard leash double-ended by putting a carabiner on the handle loop.
Comparing them, the Blue-9 is the least bulky, and most adjustable for different body shapes, but it’s not as easy to put on as the others. Ruffwear is easiest to find at local shops to try on, and they have a wide range of models for different needs. PetSafe is also available in stores. And the Baydog is easy to put on, and very ruggedly constructed. Several more brands have recently come out with similar designs, so compare whatever you are looking at with the ones mentioned here.
Regardless of which one you choose, be sure to take your time acclimating your dog to the harness, with lots of positive reinforcement. If you just wrestle it on and hope they get used to it, the dog may end up resenting the harness, and avoiding you when you try to put it on. With just a little patient work up front, you can help your dog feel neutral or even happy about putting on their harness. Ask your trainer how!