The tricky part of it all is the first couple of days. Cutting back severely on food intake, coupled with as much exercise as you can handle is key. Pump yourself full of vitamins to ensure you're getting the nutrients you need. Your body does need a modicum of fat, protein, and carbohydrates, but it can use a lot more calories from your own fat storage than you might think. When you do eat, fill yourself with tons of low or "negative" calorie food, such as cabbage and celery. The urge to eat more will be there, but fight through it. Once you get past the first couple of days, your stomach will shrink some and the cravings will lessen.
The key to it all is intake vs. output. You have to burn through a lot more calories than you take in. If you can get on an eliptical machine, for example, you can count the calories you burn much easier. Burn a thousand on the machine and force yourself to eat under that amount. That way you're guaranteed of burning a lot more than you put back in, as you're also burning calories to varying degrees throughout the day. I'll echo the swimming sentiment. It's harder to calculate exactly how much you're burning off, but an excellent low-impact way to do it.
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