First off, don't ask questions that you know your child will be tempted to lie about, such as "Did you hit your brother?" when you already know the answer. That is only encouraging them to lie. Instead say "You may not hit your brother". Of course, that is probably not a big problem with girls
Secondly, when child does say something like "The dog knocked over the plant, not me", say "I know you wish it wasn't you who did it". In this way you are acknowledging that you know what actually happened and you understand that they wish it wasn't their fault.
Anyway, the point is, young children just do this. If we adults don't let them profit from it, they won't grow up to be habitual liars. It isn't a big awful thing. I know it is scary to think of things growing to become awful habits when they are older, but this isn't likely to become one.
Good luck,
Cary