There's an old saying that goes "when you know better, you do better." That extends to everything in life, right down to how we treat our pets. These days most veterinarians are recommending that dogs wear harnesses instead of collars. Harnesses are better for most dogs, they're easier on the dog's neck and back.
Harnesses come in a multitude of styles, there is one for every dog and dog-behavior issue; whether your dog is an escape artist, a puller, a bunny-chaser, or has medical issues. Not every harness is good for every dog, but there is an appropriate style for every dog.
The first step in finding the proper harness for your dog is to measure her correctly. Manufacturers don't make dog gear by breed or weight -- they use measurements. Girth is the most important number. Take a tape measure and wrap it all the way around your dog's chest, just behind her front legs. If she'll let you -- leave it there for a moment to make sure she's not "puffing" out her ribs and snug it up if you feel it loosen. You should be able to get a finger or two between the tape measure and your dog, but that's all. If she wiggles too much, just come back a couple more times and try it again.
When you're pretty sure you have an accurate measurement, think about why you're getting Fido a harness. Your rationale will determine the type that's best for you and your dog.
If your dog has a particularly sensitive neck or issues with a "collapsing trachea," as many small dogs are prone to, look for a "Step-In Harness." These are very easy on both you and your dog, just put Fido's front legs through, draw it up over his back and clasp it. The closure should rest between the dog's "shoulder blades," well away from his neck.
If your dog is an "escape artist," hunching her shoulders and wiggling back, look for a harness with hook-and-loop closures at the neck and girth. These harnesses look like jackets and should fit closely, without much room for adjustment, or escape. They have the added advantage of being easy to put on, just lay it on the Phydeaux's back and smooth the straps together at neck and girth.
If your dog tends to pull you down the street -- try a harness with the leash ring at the front of the chest. These seem to work magically, drawing your dog into you whenever he starts pulling away.
A particularly popular harness is styled almost like a human-wear dickey, with chest coverage that distributes any pressure from the leash across the dog's chest, instead of concentrating it at the neck. These typically slip over Fido's head, one paw goes through an "armhole" and the strap is brought up under the tummy and clasped. Most feature mesh fabric and sporty styling.
There are dozens of variations and unique styles of dog harness. Materials range from leather, to cotton, to polyester. There is one to suit every dog and situation. Look for a store, locally or online, with a wide selection from many different manufacturers. Don't be shy about asking for recommendations, staffers at the shop you choose should know the strengths of every product they carry.