I was asked how I handle lying. This is a very common problem with children who are trying to gain control of their lives and other people.
In the past I have written many posts on lying. Two posts which may especially be helpful are "Communicating Honestly" And "Honesty". If you type these words into the light blue bar at the top right of this page and you will find them. In fact you will find any post that mentions honesty in it. I hope this helps. :)
In the past, with youth who are especially addicted to lying I have taught them steps to stop themselves from lying.
1. Recognize that you are trying to impress someone or get out of trouble by lying
2. Stop talking
3. Think, "What is the REAL truth in this situation?"
4. Say the truthful statement even if you have to say you just said a lie.
5. Go report to an adult that you controlled the impulse to lie.
When they would come tell me they controlled this impulse, they would earn positive consequences and praise.
Regular practice on Controlling the impulse to lie is always a good idea. The more times someone practices a good behavior, the more likely he is to automatically do it. This makes making good decisions easier for the child.
If you don't feel using the steps listed above would be helpful, then just teach the concepts and practice. Sometimes too many steps can be overwhelming. :) The idea is for your youth to teach herself how to notice a lie coming on and how to change the dishonest thought to an honest one. You want to have negative consequences for lying too. There needs to be cause and effect to teach self government.
Check out the honesty posts listed above. They have some good insights into lying.