One of the tools of discipline is a clear warning. It can actually be a teaching tool because it helps children know how to anticipate consequences of their actions. Furthermore a clear warning clarifies for your children that what you have said wasn't just a suggestion, but that you meant business.
When you give a warning, it’s important to obtain eye contact, speak calmly but firmly, and clarify both the instruction and the consequence that will come if the child doesn't respond. A clear warning says: "If you don't finish your homework you won't be able to watch TV after dinner." Or, "If you can't play nicely with your friend, he will have to go home."
A warning is different than a threat. Threats are emotional responses usually spoken out of anger or desperation with an exaggerated or ambiguous consequence, rarely leading to a consequence. "If you don't clean up these toys right now, I'm going to throw them all away!" Or, "If you don't come with me now, I'm going to leave you here!" These are threats, not warnings.
Warnings aren't always necessary. If a child hits another and you've already established a rule for such things, then it's understood that that this is wrong and you can move directly to a Break or other follow through. If you do use a warning, just give it once. Instead of a process like this: instruction, warning, follow through, some parents have a process that looks like this: instruction, warning, warning, warning, warning, explosion with anger.
Make a clear warning part of your discipline strategy and you will teach children important lessons about life and help them predict their own consequences for their decisions.