蛛丝马迹
hide or hair (通常以 "neither hide nor hair of" 的形式出现)
I haven't seen hide nor hair of the children. (or "I have seen neither hide nor hair of the children.")
　
短兵相接；肉搏战
fight hand to hand (名词：hand-to-hand fighting)
Scottish troops fixed bayonets and fought hand to hand with a Shi'ite militia in southern Iraq in one of their fiercest clashes since the war was declared.
　
混水摸鱼
fish in muddy/troubled waters
He often buys up stock in companies declaring bankruptcy; fishing in troubled waters generally pays off.
　
画蛇添足；多此一举
gild the lily
Offering three different desserts after that elaborate meal would be gilding the lily.
　
提心吊胆；焦急万分
have one's heart in one's mouth
When the plane was about to take off, my heart was in my mouth.
　
进退维谷；进退两难
between the devil and the deep blue sea; between a rock and a hard place
I'm between the devil and the deep blue sea on this negotiation; I need a car, but not at these prices.
　
虚应故事；敷衍塞责；装装样子
go through the motions
The team is so far behind that they're just going through the motions.
She didn't really grieve at his death; she just went through the motions.
　
进退两难；进退维谷
between the devil and the deep blue see; between a rock and a hard place; between Scylla and Charybdis; in a cleft stick
Trying to please both my boss and his supervisor puts me between a rock and a hard place.
Because of new employment laws, these companies are caught in a cleft stick.

附注：在这四个成语中，between Scylla and Charybdis (Scylla 是一块岩礁，Charybdis 是大漩涡) 最早出现，而 between a rock and a hard place 最晚出现 (1900 年代初期才出现)。

无与伦比；独一无二
be in a class of one's/its own
He is in a class of his own as a computer expert.

无所不知
know all the answers (通常用在 "think one knows all the answers" 这个词组中)
He always think he knows all the answers and doesn't respect anybody else's opinion.

无忧无虑；活泼开朗
bright and breezy
She is a very bright and breezy sort of girl.

无稽之谈；荒诞不经的故事
a cock-and-bull story; a tall story; an old wives' tale/story
Jack told us some cock-and-bull story about getting lost.
My sister's account of her adventures in Africa during the summer vacation sounds like a collection of tall stories.
Toads cause warts? That's an old wives' tale.

无风不起浪；无火不生烟
There's no smoke without fire
He's denied taking bribes, but there's no smoke without fire.

筋疲力尽
run out of steam
I have been working very hard on this project for months, but now I'm running out of steam.

喋喋不休 (而使人厌烦)
talk someone's head off
The woman beside me in the bus talked my head off during our journey.

闷闷不乐
down in the dumps/mouth
She's been down in the dumps ever since she lost the match.
What's wrong with him? He's so down in the mouth about everything.

焦躁不安；坐立不安
have ants in one's pants
This child just can't sit still; she must have ants in her pants.

棋逢对手；势均力敌
diamond cut diamond
They have been playing chess from morning to night, because of diamond cutting diamond.

挥金如土
spend money like water
Miss Wang spends money like water - no wonder she's always broke.

晴天霹雳
a bolt from the blue
Bill's dropping his life insurance was a bolt from the blue for his wife.

发号施令
call the shots; call the tune
It's up to the boss to call the shots.
Mary said that it's her turn to call the tune.

量入为出
cut one's coat according to one's cloth
You must cut your coat according to your cloth.

绞尽脑汁
rack one's brain(s); beat one's brains out
I've been racking my brain trying to recall where we put the key.
John's beating his brains out to finish this puzzle.

阳春白雪；曲高和寡
caviar to the general
Though more popular than it used to be, the opera is still caviar to the general.

童山濯濯
(as) bald as a coot
Nowadays more and more men are as bald as a coot.
A person who is completely bald is as bald as a coot.

恶有恶报
chickens come home to roost
Now that you're finally admitting your true age, no one believes you--chickens come home to roost.