I feel your pain. My 13year old is a lier and basically always has been. Part of it I think is because her father who she has a poor relationship has lied alot to her. He also isn't a smart lier so its extremely obvious (like he insisted I only let him have her for 1hr last Christmas despite the fact he took her to his parents place for lunch which is a 40mins drive one way so impossible for him to only get an hour when its a 1hr20min trip just to get to his parents place and he actually had her for 6hrs) I have tried alot of things and nothing has stopped it so I can't offer any suggestions except to make sure she takes responsibility for it. Its hard when she is only 8. These are some of the things I have done when my daughter lies.
For example DD13 told her friends and teachers that I have no money and she doesn't get breaskfast and sometimes she has no food for school. So she was getting money for the canteen from the school office. I had to have an interview with the school to let them know this is not true and she is telling these lies to get money. Recently she told the school I riped up her school project as I had a temper tantrum. Well yes I did throw out some rubbish she refused to pick up which was under her table for 1 MONTH which definately was nothing to do with her school work but somehow she tried to convince her ag teacher she had done it but I destroyed it. So another interview to let him know she has not done it and she has had 8weeks to do it so when its finally due and its not done she makes some grand excuse (I also told him yes I did throw stuff out and could show him what that was but it had nothing to do with her school.
Unfortunately my DD has got herself in trouble with the police for lying. It cost quiet a few thousand dollars and I did make her take some responsibility for that (as much as an 11yr old could) and unfortunately that didn't help her to stop lying. Other things she does is forge my signature which the school is now aware she will do so as a parent of a child who lies you really need your eyes open and be one step ahead of them.
Its very hard as a parent when your not a lier yourself and you have a child that lies especially when it comes at a cost to other innocent people.