The staring eye of the lens must remind them too much of the way they might stalk their prey. Having two assistants, the handler and the attention-getter, is equally important with dogs, cats, horses and all others. Puppies and kittens are the least afraid of the camera and therefore the most fun to shoot. They may not do what you want them to do, but chances are they will do something adorable. I like to shoot several together, and just let them loose if possible.
Patience is essential.
It often seems that a dog will never settle down and you might as well give up, but if you just calmly wait awhile he will, and you will get a good shot. It often happens that a dog who seems totally wild will suddenly sit down and pose perfectly. Safety is always the first consideration when working with animals. Never do anything that will endanger the animal, yourself, or anyone else for the sake of a photograph. I shoot film because the quality is still that much better than digital and I believe the quality is important.