Stupid thing to say, but match calories in with calories out – they say an average man needs 2500 cals a day to maintain weight, but it’s very rough and if you’re lifting a Hod all day or you’re 6′ 7″ you’ll need more, if you’re 5′ 2″ and sit at a desk all day you’ll need less.
If you want to be more ‘scientific’ as a very rough average under-eating 1000cals a day will result in 1kg of fat loss a week – 7000cals = 1kgs – if you know how much weight you’re losing at the moment as an average you can calculate how much you need to stay still.
Remember if you’ve got a history of over-eating (being very over weight or obese rather than just trimming a bit) you are more likely to put it back on if you don’t monitor your intake and weight – so you’ll still need a controlled diet, but it’ll just be one that matches your output.