our 13 yo has this issue as well as our 5 yo, the 5 yo goes between homes and his biggest lying issue is telling the bio mom stories about how bored he is or how everyone got more xmas presents than him, he's already learned to play the guilt thing on her...
back to the 13 yo, as there is no real punishment that suit this "crime" my husband and i have opted that each time our eldest is caught in a lie she gets a weekend of hard labor in the yard, its not the most effective punishment but man does it make her think, this perfumed, gotta be a girl 13yo who thinks she's 21 gets down right smelly after a hard days labor digging holes, cutting down trees etc....
as for the 5 yo, when/if we catch him in a lie, we require him to call his mother back (if he lied during a phone call) and apologize to her and tell her his lie (most effective concerning the i'm bored everyone got more presents recent lie)
pisses the bio mom off that we "torture" the poor child, however, we have explained, or attempted to, to her that lying is not acceptable in our home, no matter what she allows in hers.
we've had numerous talks with the 5 yo and used analogies to help him to understand that lying is bad.
its an uphill struggle but one we wont give up on, the morals we attempt to teach will eventually sink in.....
a little on the morbid/gross side, however, since our 13 yo hormones are kicking in and we've had the talk about sex, protection, abstinence, life, etc etc etc, i had her watch the TLC birthing story, and let me tell you, one look at the 19 yo crying over the contractions and the "messy" baby that was subsequently born got her squeamish enough to walk away from the tv going "oh no, no babies for me" however i did explain to her that later in life she may feel differently but i wanted her to understand the possible effects of any acts... and she does.... we've always had open communication between us and it has helped tons
she's not mad at me or anything for having to watch the show, in fact she's quite happy to have seen it to know what she does not want to go through right now as dating is just starting to peek its head around the corner.... she freely admits she wants to graduate, get a great job, have money and be able to party with her friends.....
off my soap box now.