B.H.
I strongly believe that honesty and personal responsibility are the two most important principles here. I think you have already received a lot of wonderful advice, as the mother of two teenage daughters and four younger sons I can tell you that what you are trying to do now will save you heartache later. These issues will continue to come up. Each time, just hit them pretty hard and fast and they will go away pretty quickly. When I find my girls lying to me, they have to endure a loving discussion of why honesty is so important in this world, and then they lose everything I can think of for a week or two depending on the severity. Phone, TV, Computer, Friends, Sports, etc. They know it is my job to discipline when things go wrong. I am showing them how important it is. Then life goes on and they do a great job! By the way, they are great girls. I am very proud of them, but everyone tests their limits from time to time. I just let them know where I stand.
One note about age 10, make sure that any concerns she is having are addressed. Ask her if it is because she just wants to play, or is she worried about not doing a good job, etc. Then try to help her with those concerns. She will know that you care and are on her "team", that is important around that age. Give her praise and any time she accomplishes anything, help her to see the value of the work she puts into it. That is the baby step part of the process.
I send you much strength... Mom's need it!!! :)