Busy As A Bee-
Why do people lie?
1) To escape punishment.
2) For personal gain.
3) To protect someone else.
4) To hurt someone else.
5) Because they don’t value the truth enough.
There may be times an untruth is allowed – to protect someone else’s feelings, for tznius reasons, for safety and so on.
I assume this case doesn’t involve any of these reasons.
This isn’t a magic cure-all for lying, but I’d emphasize how strongly you value the truth, how disappointed you are when told a lie, and how unfortunate it is to not be trusted because of a reputation as someone who is untruthful.
We do the above, and also stress how the lie itself is almost always worse in our eyes than the original transgression.