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Swear off fad diets

You know the kind, says Jillian: "Everyone's like: 'Give up carbs!' 'Give up fat!' 'Wait, no, now I'm taking pills!' None of them are manageable long-term—and they wreak havoc with your metabolism! Because you're either starving yourself or you're cutting out a major food group. Then you go back into weight-gain mode, but it's even worse, because your body has adjusted to all that crazy fad crap."

Next: Be an 80/20 eater