We tell our kids that if they feel the urge to gossip, they just need to ask themselves two simple questions:
1) Is it kind?
2) Is it true?
If they can't say yes to both questions, then they probably don't need to share whatever it is they want to say about another person.

Tell them a story about what would happen if they were the subject of gossip. Ask them how would they feel and what would they think of the people that were spreading gossip. By applying it to their own lives they will see that it can affect others in the same way.

I always use the boy who cried wolf scenario. Gossiping can make and break a lot of situations such as relationships and friendships. Tell your child examples of how bad it can be and hopefully this will make them thing twice before doing it.