Controlling the argument. This is a VERY important skill. If you know how to do this you'll most likely win the argument all together. If you're in control theres no way you can loose. However, having control isn't all you need to win an argument. You also need good logic. Having logic is important because you don't have time to be checking if all the facts you're saying are in fact true. Having bad logic just wastes time. It wastes your time and the other persons time. if it sounds bad then it probably is bad. Of course maintaining all this logic might be difficult for some. so just start off small.
*The following quote by aristotle is an example of great logic. nobody like criticism. The only way to avoid it is by....well just go ahead and read his wonderful logic. :)