A couple from US were invited by me to the Drama Contest held in March 2004. They said "break leg" to me just before the show of our class. Break leg? I'm quite well, no leg is broken. They puzzled and then burst in laughter. And I got to know "break leg" means "good luck".
It seems necessary to master English idioms in the course of studies. We can never become a native English speaker, but we can traverse obstacles of language so as to diminish misunderstandings and bridge different cultures. And idioms open the door to you!
The followings are some interesting idioms that I collect from reading magazines and they may make your writing or speaking lustrous if you keep them in mind and use them whenever possible.

1)Love me, love my dog 爱屋及乌
  You should love my family members as well. Don't you forget " love me, love my dog"?

2)Rob Peter to pay paul 刮肉补疮
  Why borrow to pay your debt? That's just robbing Peter to pay Paul.

3)A dog in the manger占着茅坑不拉屎
  Don't be a god in the manger. Many people are waiting for the opportunity.

4)Beard the lion in his den在太岁头上动土
  Nobody has the courage to beard the manager in his den and tell him what we all think of him.

5)Cast (one's) pearls before swine对牛弹琴
  To explain to him is to cast pearls before swine.

6)Penny-wise and pound-foolish小事精明,大事糊涂
  He is really penny-wise and pound-foolish to drive thirty miles to buy a computer for 100 yuan less than it costs here.

7)Like looking for a needle in a haystack大海捞针
  It's like looking for a needle in a haystack to find the criminal in this city.

8)Through thick and thin同甘共苦
  They've been together through thick and thin and they won't desert each other.

9)A new broom sweeps clean新官上任三把火
  As the saying goes, a new broom sweeps clean. No wonder the company took on a new look.

10)Where there's smoke there's fire无风不起浪
  There is a lot of noise coming from the office. There must be something wrong. Where there's smoke there's fire.

11) There are plenty of other fish in the sea天涯何处无芳草
  When Jack broke up with his girlfriend, I told him not to worry. There are plenty of other fish in the sea.

12)Eat one's cake and have it too欲兼鱼和熊掌
  If you intend to take post graduate exam, then you needn't try to get employed. You can't eat your cake and have it too.

13)Six of one and half a dozen of the other半斤八两,区别不大
  It doesn't matter to me which way you do it. It's six of one and half a dozen of the other.

14)A shoulder to cry on患难时可依靠的人
  I'm so blessed to have you as my friend, you are a real pal, a shoulder to cry on.

15)A knight in shining armor力挽狂澜的人
  He helped me to break away from a difficult position, and he is a knight in shining armor in my life.

Well, so much for this time. I think it easier to remember idioms than vocabulary since they are just composed of several familiar words.
Nevertheless, it's a matter of habit. Form a habit of using what you have learned, or keep it if you have already had. There's only one short cut to success of English studies----learning and using!