You should stand up nice and straight. You'll look so much nicer.
You should try to have better posture. You'll look so much better if you don't slouch.
ーYou should stand up nice and straight. You'll look so much nicer.
「背筋を伸ばした方が、もっとかっこよく見えるよ。」
「背筋を伸ばす」は stand up (nice and) straight とよく言います。
ーYou should try to have better posture. You'll look so much better if you don't slouch.
「姿勢をもっとただした方が良いよ。背中を丸めないともっとかっこよ見えるよ。」
have better posture で「より良い姿勢をする」
slouch「前かがみの姿勢」
ご参考まで！
カナダ 
