You can make your stomach look flatter instantly by improving your posture.

你可以改善坐姿，腹部就会马上显得平些。

《柯林斯英汉双解大词典》

Also, your posture is key.

同样，你的姿势是关键。

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Check your posture once in a while.

偶尔检查你的姿态。

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Be aware of your posture, your facial expressions and your gestures from the moment you arrive.

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To others, you'd look stuffy if you didn't lean in to engage in conversation because you were more concerned with your posture.

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Exude confidence in your speech, your posture, and your body language to indicate that you are not someone who can be taken advantage of.

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