I agree with Kay, and I've had a chubster that desperately needed to lose weight because he kept getting urinary blockages that are more likely with overweight cats. Because the stakes were so high (the blockages were life-threatening) I was very, very strict and he did lose a kilo over about 8 months, thankfully. However, it was really, really tough and please don't think that I'm suggesting that anything I'm about to say is easy!!
Firstly, no feeding on demand. Ever. Set up a feeding routine and stick to it. I've had several ex-strays that were hard to convince that there would always be another meal at the next feeding time and would cry for food as though starving if they thought there was the slightest chance of it getting them anywhere. When they realised it didn't achieve anything, they stopped. I think cats sometimes cry for food just because they're bored or want the attention. Sometimes they're just a bit peckish!
I cut down on his dry food, as the dry is much more calorific than the wet. I don't have a huge amount of faith in it, but I also put him on the reduced calorie version of his urinary prescription dry food - I figured it had to be worth a try. And excercise, I tried to make sure he was more active with play and encouraging him to go out, though that backfired a bit when he started stealing sausages from neighbours' barbeques!
As Kay says, don't expect much in the way of cooperation! Good luck!