There's only one way to lose weight safely: burn more calories than you eat. You're not going to convince your cat to get on the treadmill, so it's up to you to increase his activity. The most reliable method is for you to exercise him with daily play sessions. Try different toys to figure out what he likes. Just a few minutes of chasing after a catnip mouse, for example, can make a difference.
A weight-loss diet means cutting your cat's calories and then maintaining the right number of calories to keep him at his lower weight. You should also cut out snacks and treats, including people food. Be ready for some protests at first. To get your fat cat on the road to fitness, however, you have to feed him less. Many owners unknowingly overfeed their cats, and the feeding guidelines on pet food containers aren't going to work for every cat's metabolism. Ask your vet exactly how much your cat needs to eat each day to lose weight (and maintain it), then be sure to measure it out each time you feed him.
Low-cal cat foods are designed to make your cat feel full while actually giving him fewer calories. In some cases, your vet may prescribe a weight-reducing food or suggest buying a commercial brand. Make sure there's plenty of fresh water available, and ask your vet about feeding your cat both dry and canned food -- the latter can keep him fuller, longer.
Avoid "free feeding" your cat. You can't tell how much your cat is eating if you leave a bowl of food out all day. Keep meals at regular intervals, and pick up what isn't eaten after about 20 or 30 minutes. Your cat will learn to eat when the food is out. If you have more than one cat, you may need to feed them in separate rooms so that they'll stick to their own bowls.
If your dieting cat gorges on his food and then acts like he is starving later, try splitting the food up into smaller meals. Also, keep in mind that your cat may actually be craving attention, not food (although he will take food instead). Try petting or playing with him first.
If your kitty's weight-loss program doesn't seem to be working after several weeks, you should definitely call the vet for advice. However, if you follow these guidelines diligently, your cat should soon be slimmer, trimmer and healthier.