Being kidnapped and held for ransom by Somali religious fundamentalists usually does the trick.
In your teen years your body burns fat faster than any other time in your life. Regular exercise & a healthy diet is all it takes. Call it old fashioned but if it's worked for this long there must be something to it.
eat proper size portion meals and exercise 30 mins a day... same way doctors have recommended for the last 50 yrs
It is easy if you stick with it for few weeks as drink much of water...eat fruits and vegetables with less fats like no more fast food... use salad as a way to eat when feel like hungry and put olive oil on salad whenever you eat so to make your body always fit. Swimming as well or walking 15 minutes a day will do the job for you. Taking stairs like daily at day and night for few times like 10 times at each period. If need any help let me know and you didn't tell about your height and weight right now and please weight in kilos :).
Date a vamp or lycan.
Cheesy as it sounds, but eat all of the food grounds, eat in moderation, eat a bunch of small meals/ snacks a day instead of three big ones, and exercise !!
Don't eat anything until your parents throw you in the hospital or an eating disorder clinic!
Exercise, eat less fattening foods, and drink water.
Run / Job, avoid saturated fats and sugar
Proper diet and exercise.