I don't have much experience with kids this age (yet), but I remember doing this as a kid.
What I remember was being scared of my mum's response if I told her the truth. So say I'd done something not really thinking about the consequences... if I admitted to it then I'd be told of/punished/my mum would be really disappointed with me when I already KNEW that what I'd done wasn't the best choice. I'd lie in an attempt to avoid that, and I kept doing it because at times it worked.
I know it used to infuriate my mum... and she'd talk about how honesty was most important etc. I still used to feel like it was my mum's fault, because the way she acted meant I couldn't tell her the truth about things.
No advice, I have no idea how to respond to that which might be effective, but that's how I remember looking at it as a kid.