In addition to reducing calorie intake, it is important to increase the calories used. Exercise, may in fact, be more important than feeding a diet food.
Exercise programs will need to be tailored to the dog taking into account the condition of the dog's muscles and joints, heart, and respiratory system. It is important to choose activities appropriate for your dog and do not overdo. Start slow and work up to higher activity levels. Rest if you notice signs of fatigue, like heavy panting. In general, leash walking for 20-60 minutes a day, five days a week would be a great way to start. Swimming is also an excellent activity for obese dogs, since it places less stress on joints.
Exercise is a great way to give your dog attention and can be a substitute for treats. Exercise will help your dog build muscle, and increase mental stimulation, taking his focus away from food.