A passage I read from 'The Last Lecture' by Randy Pausch which I want to share (:

48. Tell the Truth

If I could only give three words of advice, they would be "tell the truth." If I got three more words, I'd add: "All the time." My parents taught me that "you're only as goof as your word," and there's no better way to say it.

People lie for lots of reasons, often because it seems like a way to get what they want with less effort. But like many short-term strategies, it's ineffective long-term. You run into people again later, and they remember you lied to them. And they tell lots of other people about it. That's what amazes me about lying. Most people who have told a lie think they got away with it...when in fact, they didn't.