When it comes to doing the right thing, we have all heard people say "it depends on the situation." I'm convinced that's false; it's a trap, it's a temptation to justify the wrong we are about to do.
You cheat on your wife. Do you tell her? Yes.
You lie on your resume. Is that okay? No.
You accidentally ran over the pet dog and killed it. Do you tell the owners? Yes. Do you tell your kids a different story? No.
Upon reflection, everyone agrees that lying, for example, is wrong in most situations. Honesty is the best policy. Of course we could make up silly little hypothetical scenarios where lying would be okay, and that would be jolly fun, and that's what freshmen philosophy majors are best at - but let's get real for a minute here. Moral decisions are made in real time, in real life, involving real people. When is lying actual okay?
How about never. Smoke that in your pipe, son. Let's think about the psychology of lying. It's not until people get into hot water that they consider lying to be a viable option. That should be an immediate hint: it's wrong, whispers the angel on our shoulder. But, when emotions are involved, we don't think right. "Well," we say, "in this situation, lying is okay because of x, y, and z." We calculate, add, subtract. But that's backwards. (1) We do something stupid. (2) Then we lie. (3) Then we justify. Coward! Your situation is not special. You are not more special than anyone else.
How about this: (1) we rationally conclude that lying is wrong. (2) We do something stupid. (3) We tell the truth. Even better: we don't do something stupid in the first place. Morality, after all, is good habits of thought followed by good habits of action. The goal of morality is to live such a virtue-filled life that vice has no place, no time, no situation to live in.
Jesus might have said something like this: You are only honest when the cost is low. Even sinners do that! Be honest when the cost is great. Then you will know sacrifice and forgiveness. Now live in honestly and truth: now you enter the Kingdom of God.
Lying Contradicts the Idea of Communication
Everyone deserves the truth, including yourself (we lie to ourselves perhaps most of all). The fundamental purpose of communication is to tell the truth, to share information. If you think about lying abstractly, you realize that it goes against the very core of human communication; it explodes the whole system. It's an exception to the rule which breaks the system. Immanuel Kant saw the absurdity in lying and all other irrational vices. He had the brilliant, simple mind to see that morality involves a few, simple, rock-solid principles that should be followed: never lie, never cheat, never steal, never hurt people, etc. And the means do not justify the ends. Day-to-day morality doesn't have to be so complicated. He simplified morality into a test: do not act on those principles that cannot be universalized for everyone. Everyone can't lie; therefore you can't lie. We are all legislators in the same moral community. Don't be the dick head with special interest groups writing your legislation and ruining our lives.
Now, I don't want to sound like morality is so simple all the time. But I do think it's simple most of the time, especially when it comes to negative morality (the "thou shalt not" stuff). However, sometimes we really do find ourselves in a moral predicament. Sometimes our values clash and compete with each other. Good luck!