A good beginning makes a good ending.

善始者必善其终。

A wonder lasts but nine days.

新鲜事儿不久长。

Christmas comes but once a year.

佳节良辰，机会难得。

Constant dropping wears away a stone.

滴水穿石。

Good fame is better than a good face.

美名胜美貌。

He is a good friend that speaks well of us behind our backs.

背后说人好话的够朋友。