金科玉律
golden rule
I have one golden rule that anyone can put into practice: eating less makes you healthier.

 
空中楼阁
castles in the air
Your plan to run for the president is merely a castle in the air.

 
知足常乐；足食犹如盛宴
enough is as good as a feast
The old proverb "enough is as good as a feast" doesn't apply to her.

长话短说；简而言之
cut/make a long story short
To cut a long story short, the fact is that you are not welcome here.

 
来日方长
tomorrow is another day
After he had failed the GEPT, I told him that "Don't be too sad. Tomorrow is another day."

 
卑躬屈膝；毕恭毕敬；点头哈腰
bow and scrape
From the way she's bowing and scraping in front of the boss, you'll know what sort of person she is.

事后诸葛；事后聪明
wise after the event
If we had waited another week we could have bought the car more cheaply. Well, it's easy to be wise after the event.

 
直言不讳
call a spade a spade; speak one's (own) mind
I like this guy because he always calls a spade a spade.
I'm furious about it, and I intend to speak my mind to my boss.

 
枉费心机 (或白费心机)；徒劳无功；狂犬吠月
bay (at) the moon
This company has spent a lot of money on advertising its new improved type of CD player. But it's baying at the moon. Who wants a CD player when DVD players are now cheaper?

附注：bay 在此当动词用，意为「(猎犬) 不断地吠叫」。

孤注一掷
put all one's eggs in/into one basket; put one's shirt on
When the company she had invested all her money in went bankrupt, she wished she hadn't put all her eggs in one basket.
I think I've a very good chance of getting that job, but I wouldn't put my shirt on it.

乳臭未干
(still) wet behind the ears
He made a lot of mistakes, back when he was still wet behind the ears.

 
屈指可数；寥寥无几
can (or be able to) count on one's fingers
He's too stingy, so he can count his good friends on his fingers.

迎头赶上(或摆脱困境；挽回损失)
make up leeway
Despite the fact that John has poor performances in exams, the teacher has decided to give him a chance to make up leeway.

奄奄一息；半死不活
(to) within an inch of one's life
Her husband beat her to within an inch of her life in a domestic violence. (她丈夫对她家暴，把她打得奄奄一息/半死不活)
附注：这成语的基本型式为"within an inch of" (= very near, very close to：差一点，险些儿)，如："We came within an inch of death." (我们险些儿丧命)。

 

 
明珠暗投；对牛弹琴
cast pearls before swine
I tried to explain the beauty of Chinese characters to my foreign friends but it was just casting pearls before swine.

怏怏不乐(情绪或心情不佳)
out of humor (in a bad temper; moody)
He seems out of humor. He has been shouting at everybody all day long.

昂首(或抬头)挺胸；趾高气扬
hold one's head high; with one's head held high
I have boasted in my youth and held my head high and gone on my way careless of consequences. (Evelyn Waugh) -- 伊利文渥夫为英国作家，生前着有「一掬尘土」(A Handful of Dust) 等作品。
It is important to walk with your head held high to keep your body shape.