Do as you would be done by.

己所不欲，勿施于人。

Doing is better than saying.

与其挂在嘴上，不如落实在行动上。

Do it now.

机不可失，时不再来。

Do nothing by halves.

凡事不可半途而废。

Don't claim to know what you don't know.

不要不懂装懂。

Don't have too many irons in the fire.

不要揽事过多。

Don't make a mountain out of a molehill.

不要小题大做。

Don't put off till tomorrow what should be done today.

今日事，今日毕。

Don't put the cart before the horse.

不要本末倒置。

Don't trouble trouble until trouble troubles you.

不要自找麻烦。