KZ and PG have great advice!
Im actually battling obesity with my cat right now. Try to cut out the dry food entirely and if you do give treats use them in one of the toys like PG mentioned or as a reward after physical activities like a good play session. That way the cat gets good exercise and you get to have that quality time together. Of course, make sure to limit the treats to a conservative number per day:).
Aside from those things, what has worked for me is carefully observing portions (cats really dont have to eat all that much) and switching to a raw diet. Nothing else was working, not even switching to wet food. It takes a lot of research but if you do it right, your cat will be happier, healthier and if you make your own, its very economical. Its been a slow road but now instead of 16lbs, shes at a happier 12lbs.
Play, portions and observing your pets diet are essential to weight loss in kitties!
Also, KZ beat me to it and posted the website I was going to recommend :P. http://feline-nutrition.org/ Great site!
Good luck with your chubster!