Don't give in to the temptation to argue or have the last word with your child.
Kids invite us to argue with them all day long. The type of argument I find myself in most often comes when a child asks over and over again to have or do something. I have already given them the answer: "No, you can't have a cookie right now" or "We are not going to the Library today." Yet they do not take no for an answer and ask again and again and again.
So I have begun training my 3 year old that I answer questions once (grace is gladly given if you simply have a bad memory!). I use a sweet voice when I am training, trying hard to not be sarcastic or show frustration/impatience and I say, "Oh, I already answered that question, huh?" and they ask again and I say, "Mommy gave you the answer to that question already. Do you remember what mommy said?... that's right! Good memory!" and they ask again and I might say, "I have heard that question already. Do you have any OTHER questions for me?" I try to rephrase the same basic point again and again, CAREFUL to not accept the invitation to answer the question again. To accept this invitation is to enter into an argument!
Kids invite us into arguments all the time but we might not realize that we mothers send out invitations, too. One way we invite arguments with our children is that we hang around to make sure they actually do as we have asked them. As Diane Moore once said, be sure you keep yourself as far away from the lawyer's table as possible. A lawyer makes a case for his side of the story and then sits down, ready to hear the rebuttal. A judge, on the other hand, says his peace, drops the gavel and then leaves. Even if you felt like arguing the decision, you can't because he has already left the room.
So, Mom, ask your child to clean his room, "I'll see you in 20 minutes. That gives you plenty of time to get the job done right." AND THEN LEAVE. Don't stand over then waiting to see if they obey and harping on them the whole time. Just leave. And then come back ofter 20 minutes just like you promised and congratulate them on a job well done, or dole out the appropriate consequences for the disobedience.
Another reminder I give myself is to use those wonderful Love and Logic phrases when kids are really upset with us. They suggest using short simple phrases with as little energy and emotion as possible that give you something to say that will not escalate the argument. Like when a kid yells furiously at you, "But Mom!! That's not fair!" You can stay away from Lawyer-land by saying things like, "Probably so," "I don't know" and "I guess."