atkins?
Make it run on the treadmill?
I think you get a cat to lose weight the same way a human does... exercise more, eat less... Portion control on it's kitty chow... (measuring out her portions) Watch between meal kitty treats...
When she goes to the vet next, ask the doctor if her weight is normal? my parents had a cat that was huge... cat wasn't fat at all - he was lean and mean.. he was just big... if you feel around her tummy is she pudgey or do you feel muscle there?