The best way to lose weight is to feed less or feed a low calorie diet like Hills canine R?D, available at the vets. There are also new drugs like Yarvitan that can help. I prefer to check your pet for Diabetes, Cushings or Hypothyroidism to exclude medical causes.
Most low calorie dog foods available in pet shops do not work.Excercise will also help. For cats the best device is an electronic fishing rod which wafts about exciting your cat making it want to catch the bait which moves at random.