保持身材的英文翻译是"Keep fit"。
例句：
1、我一直注意保持身材。
I have always paid attention to keeping fit.
2、她的新年决心是要保持身材健美。
Her New Year's resolution is to stay fit.
3、他经常运动，以锻炼身体和保持身材。
He exercises regularly to stay in shape.
4、她要保持身材苗条，就不得不节食。
She has to watch her diet to stay slim.
5、很多人希望保持身材苗条，但同时又担心自己的健康。
Many people want to stay slim, but at the same time, they are concerned about their health.