The number one rule in breaking down the liar will be exercising patience! Getting them to come clean will not guarantee a change in their behavior, but you can push them to the point of truth for your particular situation.
They have always said you have to be smart to be a good liar and let’s face it, compulsive liars lack intelligence. They have trouble keeping their stories straight and as you call them out they swear they weren’t lying. They swear they didn’t say something they have said in attempt to cover the lie prior to the bust. They get defensive, and often storm off. After they have cooled down, push some more. Keep forcing them to face it.
In some cases you may need to pretend to know things you do not know. And in every case that involves an compulsive liar, there will be a lot of homework that needs to be done. As you discover things pertaining to your situation, encourage him/her, and tell him/her how much better he/she will feel coming clean. Let him/her know how much better the relationship will be once the truth is out.
It is also important that you show little emotion when they do open up. If they sense anger, they will stop opening up and revert to the lying cycle. Encourage the change, but also know your limits. Compulsive lying is an addiction, and as any addiction relapse is always possible. If you love this person, keep doing your homework and keep calling the person out. Often times if a compulsive liar gets caught up a lot, he/she will at least reduce the amount of lies, and sometimes they even stop completely.
If the relationship is weighing you down, maintain your distance. Sometimes the only way people learn their lessons and change; is when a loved one walks away. It forces them to hit rock bottom, and only then do they begin to live a more honest life. Some people will not learn until they have suffered the consequences of their actions.