(Sigh)
My personal rule: Only lie when it's necessary to protect someone from undeserved harm.
Obviously, since it's my rule, and word "harm" can refer to a lot of different things, there is quite a bit of variability in what I consider to be an acceptable lie.
But I would estimate that 99% of the time I tell the truth, very few situations come into play where my rule has to be used.
Here's the thing, lying is largely used for three purposes:
1. Manipulative: To make other people feel good,ellicit a desired response from an individual i.e. pity, sympathy,anger, ect.
The problem with 1.: This kind of lie works well in the begining, however, this kind of lie is also frequently used on people that you have close contact with, and will have future contact with. It's also the second easiest lie to detect. It is inevitable that your lying will be discovered, at which point ( even if you are never confronted about it) people will cease to trust you, like you, or take you seriously. I know several people who constantly lie in this manner. For example, I know one person who does this in an attempt to further his career. What he doesn't know is that people are always laughing at him behind his back, and his reputation has extended into other departments and even other related companies. With the exception of people new to him, and extraordinarily stupid people, he is a walking punchline, in his case, he will likely never get ahead in his career.
2. Defensive: A lie employed exclusively to get yourself or someone else out of trouble.
The problem with 2.:This lie can be applied to my rule above, so obviously I think it has a potential justification. Unfortunately, most people use this lie far to frequently to get any benefit from it. This lie should only be used in the most extreme cases, where the truth will ultimately cause extensive and unwarranted damage. Classic example: A man steals bread from a store to feed his starving family because he has no legal means of aquiring food. If asked if he stole the bread, the man is justified in lying, because if he tells the truth he will go to jail or at the very least the bread will be confiscated and his family will still starve. Most people fail to use this lie effectively by using it as a "get out of jail free card" for literally anything that they do wrong.
3. Ego: A lie employed to boost your own public standing, or designed to boost your own self esteem.
The problem with 3.: This lie is closely tied, and often used in conjunction with lie type 1. As such, it is the easiest to detect. This particular lie has been used in plays, movies, and television shows to demonstrate stupidity in a character or teach a moral lesson. It has no justifiable value, and often creates loathing torwards the person who employes it. Politicians, buisness men, and other "respectable" positions often attempt to employ this lie successfully. The popularity of this lie is largely due to it's risk/reward status. If you manage to use it successfully, it can have huge benefits, fail and you will likely lose everything. Only the most proficient liars have been able to use this lie, and most of them are so good at lying that even if they are caught, they simply employ a new lie type 3. to recover from the loss.
Here's a pro-tip: Lie types 1 and 3 can be accomplshed almost as easily with the truth. The truth has very little risk/reward status, and people typically respect and remember when you tell the truth about things related to lies type 1 and 3. Which means that as you progress through life, you build a reputation, and people will begin to come to you because they know they can trust you. You can build lasting friendships and often reap rewards because of it. Think about the person you trust the most. . . Why do you trust them? I guarantee it is because if you have seen them lie, it was only to use a varation of my rule, and also because you have never seen them employ lie types 1 and 3.