I've found that literally stopping to take a breath is actually a very good practice for this situation. Helps me avoid a lot of bad moods caused by inconsequential things. Process the negative information, realize it's inconsequential, and return to a state of calm.
(I don't count to ten)
This applies for small things, but most things that people get distressed about are small.
It's amazing how well this works, at least for me.
Sometimes I even make an effort to think to myself, "How would X respond to this situation?" Replace "X" with anyone of your choice who would probably be a lot cooler than you under stress: Jesus, Spock, Batman, James Bond, Yoda, Mr. Rogers, or whoever the heck epitomizes "chill" in your worldview.
It sounds cheesy, and it probably is. But it works. Chances are, the "X" of your choice wouldn't react hastily or let small things get to him. Shit happens to everybody, but stopping in mid-emotional-flareup to process an appropriate response is a skill. And skills can be developed.
(I don't count to ten)
This applies for small things, but most things that people get distressed about are small.