Instructions
Travel to the marketplace in the town of Bowerstone and go to the Bowerstone Produce Stall (called the Bowerstone Fruit and Veg stall in non-American versions of the game.) The address is 7B Market Square, Bowerstone.
Purchase as much celery as you can afford, or as much as the Produce Stall has. There are several types of celery, rated according to quality. Every piece of celery reduces your character's weight by 5 percent regardless of its quality, so if you're trying to save money, buy nothing but the poorest-quality celery. Note also that the more highly-ranked celery increases your character's purity, changing his appearance, so you should also go for poor celery if you're trying to retain an impure character while losing weight.
Eat the celery to bring down your weight. If you bought out the market's celery supply, come back the following day, when they should have restocked.
Keep the weight off by paying attention to the "fatness" variable on every piece of food you eat. Don't eat foods that add a lot of fatness. If you want to lose weight again, you can always come back for more celery -- it's the only food in the game that decreases your weight.