lipo or situps
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Sounds like you're out of shape.

If you're only 16, but you have a flabby stomach, you're in bad need of exercise. Nobody likes to hear this, of course. You'll probably ignore this suggestion, so I don't know what to tell you.

Eat more fruit and vegetables so your skin is healthy. You'll shrink back up.

But you need activity. Exercise. Take up jogging. Kayaking. Anything.
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Sounds like you're pretty skinny, so i wouldn't worry about it- but if you really want to try for a flat stomach, try doing crunches or other ab-exercises. You also might want to try pilates, they gear towards strengthing the core muscles (abs)
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Crunches crunches crunches, oh yea and then more crunches…. Im a mother of four, and have a flat stomach….they work wonder's, whether or not you have kids or not.
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http://www.fitness-ebooks.com/

Follow this workout twice a day, everyday…

30 leg raises with light ankle weights
(Lie flat on your back and lift your legs untill your feet are pointing to the celing, keeping your legs completely straight at all times.) Thisis very effective and work your lower stomach musles

30 sit ups

30 crunches

Do no more than this though! Your musles need time to recover and going over board can cause injury.

Drink a lot of water as well.

Also, cosmetics can help your stomach too. I use a body scrub, with exfoliating gloves in the shower. Then when I get out, I brush it with a body brush, this improves circulation. ( you can use it on your thighs for celulite too.) Then apply an anit-celulite gel on your stomach rubbing in circular movemets. Then finish of by using a firming body lotion. Hpe this helps
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I'm a personal trainer