Paper vs Reality

on paper
書類上

turn out well
うまくいく

Example:
The idea looked good on paper, but it didn't turn out so well in the end.
書類上ではいいアイディアだったのに、結局うまくいかなかったな。

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count
重要である

Example:
It's what's on the inside that counts.
大切なのは中身です。

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Comment:
That's why you can't judge a book by its cover.

Uphill Battle

uphill battle
苦しい戦い

Example:
It looks like peace in the Middle East is going to be an uphill battle.
中東での平和が困難な闘いになりそうだな。

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Comment:
The world would be a better place if only it wasn't so hard...

Rules

follow the rules
規則に従う

Example:
If you don't follow the rules, someone'll get hurt.
ルールを守らないと誰かがけがするよ。

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play by the rules
フェアプレーする

Example:
The game's no fun if you don't play by the rules.
フェアプレーしないと楽しくないよ。

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Comment:
You can use this expression even when it's not a game, in which case it means the same as follow the rules.

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bend the rules
規則を曲げる

Example:
I bend the rules sometimes when I have to.
必要な時はルールを曲げる。

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break the rules
規則を破る

Example:
Rules are meant to be broken.
ルールは破られるためにある。

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Comment:
No they're not, let's all follow the rules like good people...

Big Deal

big deal
大したこと

Example:
It's not such a big deal, so why do you have to bring it up all the time?
大したことないのに、何で一々その話を持ち出さなければいけないの？

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Comment:
I don't like people that make a big deal about everything, like my parents. Simple is best.

Smoking gun

smoking gun
煙が出ている銃

Example:
Comments are dead. Twitter holds the smoking gun.
日本語出てこない。

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(Quote by Dana Oshiro from ReadWriteWeb)

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smoking gun
決定的証拠

Example:
We never did find a smoking gun in Iraq.
イラクに決定的証拠を結局見つからなかったな。

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Comment:
The term smoking gun was used a lot by the media in the run up to the Iraq invasion.

Listen up

listen up
よく聞く

Example:
Listen up. I'm going to announce the results now.
よく聞けよ。これから結果を発表します。

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Comment:
This phrase is really only used as a command, as in the above example.

Idioms

Found this cool page at http://www.listen-it.com/ It focuses on English idioms, so it may be a little difficult, but the explanations are really good, so I highly recommend checking it out.