You know it's time to change collars once you have noticed how Bullet's head looks smaller than his neck and you have repeatedly witnessed him backing out of his regular collar. While it's true that a martingale collar can be helpful to prevent Rover from slipping out of his collar, it's also true that for the collar to be effective, you must learn how to ensure the perfect fit.
Familiarize yourself with the martingale collar. Also known as a limited-slip collar or greyhound collar, the martingale collar consists of two loops. The larger loop is the part that goes over the dog’s head; whereas, the smaller control loop, is the section that contracts when tension is applied, preventing the collar from slipping over the dog's head. The leash attaches to a D ring found on this loop.
Measure the widest point of your dog's head, which is normally around the back of the head from under the chin to the front of the ears. This length will help you determine if the collar will be able to pass over the head. Next, measure around your dog's neck where the collar will rest. These two measurements will be helpful for sizing the collar correctly.
Shop for a martingale collar that matches your measurements, but don't forget to add an inch or 2 otherwise the collar risks being too tight. Generally, it's best to look for a collar size where the measurements fit somewhat in the middle. Martingales are fully adjustable collars so they can be tightened and enlarged as needed.
Fit the collar on your dog. You'll likely have to enlarge it to get it over your dog's head. Once over the head, adjust it so that it rests in the middle of the neck and you're able to easily slide a finger under. Attach the leash to the D ring. Make sure the two loop buckles never touch as this means the collar is too loose. Expect there to be about 2 inches of fabric between the hardware pieces.