Clothes are a swing issue. Character, on the other hand, is a crucial issue. How a child dresses, how he decorates his room, are swing issues. But lying is a crucial issue because it has to do with developing godly character. Determine whether your issue is a swing issue or a character issue, and consider the age of your child.
Timing is important.
When you say something can have a dramatic impact on how it's received. Let's say the mail comes, and you discover your son's received a notice that he bounced another check. Then you notice him coming in from school. Resist the temptation to hit him with your lecture the minute he walks in the front door. Instead, welcome him home, give him a snack, and hear about his day.