Guiding children's behavior through rules and limits is a big part of parenting. At some point in their development, children will experiment with you to see just how serious you are about those boundaries. That's why parents need to establish and clearly communicate the consequences of breaking those rules.
Here's the problem: Parents don't really want to punish their kids. It's so easy to think that a warning (or two or three) will avoid a fight, save everyone's feelings and fix the problem. Instead, failing to enforce the consequences of bad behavior just makes your child see you as unreliable and easily manipulated. And since engaging in the bad behavior carries no consequences, your child has no reason to change it. In fact, your child's behavior may become worse if not appropriately disciplined. Kids want the limits, and they'll probe until they find them.
If you want to change someone else's behavior, the best place to start is by changing yours. Set the limit, communicate the consequence and then calmly follow through when your child steps out of line. Be sure to create consequences that have meaning for your child -- like taking away a favorite toy for young ones or a cellphone from older kids -- and that you're willing and able to enforce consistently. Consistency is important when you're trying to change your image as an unreliable parent. But there will be outrage when you first enforce the consequence. After all, from your child's point of view, if you didn't mean what you said last time, why should you mean it this time?
In another scenario, parents may tell their children they'll do something for them or with them, and then don't. Both of these are examples of failure to follow through. The first deals with discipline; the second is a broken promise. The outcome, however, is the same. When you don't do what you told your child you would do, you become someone he or she can't rely on.