Summarize:

More activity burns more calories. An active cat will lose weight much more effectively than a cat that never gets any exercise. In addition to the extra calories burnt, exercise for your cat will also reduce her appetite and increase her metabolic ratewhich can counter the slowed metabolism related to reduced-calorie diets. Engage in a game of chase by projecting a laser pointer on the ground and having your cat chase it. You can also use toys attached to string to play with your cat and get her burning calories. If you have trouble getting a lazy cat up and running, then try a catnip toy. Water is important in keeping your cat active as well as aiding metabolic function. As with people, drinking fluid can also assist in making your cat feel fuller while on a restricted diet. Ensure that you keep her water dish consistently clean and full. Like people, cats can eat out of boredom. Many cat owners make the mistake of providing attention mostly in the form of providing food and treats. Instead, spend more time playing with your cat and brushing her to cut down on potential boredom. Do not place all of your cat's food for the daywhether a diet formula or otherwiseout at one time for your cat to eat freely throughout the day. Cat's do better with controlled portions provided at intervals, so you should divide your cat's meals into two or three servings a day. You should cut back heavily on the amount of treats you're giving your cat since these amount to empty calories. Instead, use play, grooming, and other attention as positive reinforcement. Another behavioral strategy you can try is to get your cat to mimic hunting behavior. A feral cat living off mice makes a kill three to four times a day, and is used to having several small snacks throughout the day. To mimic hunting behavior, fill several small food bowls and hide them around the house. This will force your cat to go 'hunting' and exert some energy to try to find her food bowls. This faux hunting helps to engage your cat both physically and mentally. Another option is to use a play ball filled with food. These sort of balls require the cat to play with the ball and bat it around in order to make the kibble fall out.
Exercise your cat. Provide plenty of water. Entertain your cat. Provide a higher frequency of smaller meals. Reduce treats. Use your cat's hunting instincts.