LEATHER LEASHES
There is one training tool that is always consistent in dog training no matter what type of dog you are teaching or the animal's personality, and that is use of a leash. It is essential that you always make sure you have a leash on your dog when you are training him. The reason is that it gives you a lot more control and it also gives you the ability to keep him where you want. Think about it. If your dog is not on a leash, you have absolutely no control during the training session. Without a leash on your dog, you will constantly find yourself in a position where you are trying to reach for him, calling him, or repeatedly telling him to come to you.
As you are shopping for a leash, the one thing you will find is that they come in almost every size and color. I always recommend using the length and type of leash based upon the size and personality of the dog. In most cases, just a 6 foot leash will work fine. But remember that there are different widths of 6 foot leashes, and they come in everything from a cheap nylon leash (which can work fine in many situations) to an expensive leather leash (for dogs that are wanting to pull). But it all really depends upon the type of dog, and the size of dog you are teaching.
If you are teaching a Jack Russell Terrier or a smaller dog that is really wanting to pull, you probably are going to be using a very narrow 1/2 inch - 6 foot leather leash. Remember that this is the type of dog that is most likley going to want to pull on the leash, so you are probably want to use something that is going to be strong and easier on your hands.
You can follow this same idea with larger dogs too. If you are teaching a very high strung Labrador Retriever or any other large dog that is out of control or simply going to want to pull, my suggestion would be something strong and that is easy on your hands.
A great rule of thumb is to take a look at my dog training book "What Color is Your Dog?". In general, dogs that are orange or red are going to be pulling and jumping, and you are going to need something gentle on your hands that is also strong. I would highly recommend a leather leash for orange and red dogs.