I think by making it clear that if they lie the punishment will be MUCH worse. I really like the strategy of putting the choices back on the kids. "By choosing to lie you are choosing to not have access to the car for a week" or whatever consequence. Make sure they know the consequences for their actions, and make sure it is massively worse if they lie. Grounding doesn't work unless you make it boring, basically turn her into the house slave and take away tv, computer, cell phone privileges so she has nothing to do. Or even sign them up for community service (if it's a teen).