This idea goes along with the book Alexander and the Terrible, Horrible, No Good, Very Bad Day and comes from one of my college professors. In his room, he had a table reserved as "Australia". If one of his students was having a bad day they could go work at the Australia table for a few minutes until they were feeling better.
I've also found this to be a great tool to tell parents about at conferences. If a parent happens to mention that their child is feeling upset or frustrated with a classmate and feels as if I should be noticing said frustration, I tell them all about the flags and ask their child if they've been using them. I'm not a mind reader, but I can see a flag if you slap one on your desk.
If you are short on time and don't want to make your own flags/poster, check out my TpT product that includes 8 different posters and 3 different styles of flags. Just print and go!