A wonderfully straightforward piece of advice from ‘Mrs Hawkins’ in Muriel Spark’s A Far Cry from Kensington:
As an aside, I can tell you that if there’s nothing wrong with you except fat it is easy to get thin. You eat and drink the same as always, only half. If you are handed a plate of food, leave half; if you have to help yourself, take half. After a while, if you are a perfectionist, you can consume half of that again … I offer this advice without fee; it is included in the price of this book.