I have been told I have a very good poker face by my boss as well as others (I'm a poker player so good to know it works on most people). I think just like anything else it takes practice and being in stressful situations. That said, there are some steps to it as well:
1. Hear or see what is being said and do NOT react. An automatic reaction is failure to keep yourself disciplined and composed. Read the article on stoicism on this site here:
http://www.mrmoneymustache.com/2011/10/02/what-is-stoicism-and-how-can-it-turn-your-life-to-solid-gold/2. Internalize what is happening. Give your response some thought. You are an adult and have mastered how good responses sound, and most importantly, what a bad response sounds like (i.e. talking frantically, not hearing what the other person just said, etc).
3. Take a deep breath. You are doing this for two reasons: one is to slow your heart rate/anger, the second is to allow step 2 to take place.
4. Give your response as is appropriate with logic. Just the fact that you took half a second to think and slow your response down automatically makes your response more reasonable.
When you think about anger, it is one of the most short-lived emotions, but it can cause your voice to crack, your words to become jumbled in your head, or you to become impassioned to your side when maybe you are actually in the wrong. Let reason and a clear head prevail. I do this all the time and it is second nature to me to the point that almost no one has ever seen me angry (a close friend saw me truly angry once years ago and still talks about that one time, because it just never happens).