Pro tip: DON'T!
Don't try to cheer someone up with the usual crap like "it's a game shit happens the next one will be better" and whatnot. It'll only make things worse. Instead, let your teammate take a moment to relax and get their bearings. Let them cool off on their own terms. Telling someone to cheer up after a fuckup, can seem patronizing especially when your mind's clouded with anger.
I'm sorry, but can you define the term "Cheer Up" for me? It sounds like some kind of new concept.
What is this from?
I just sing them this song https://www.youtube.com/watch?v=uXnFhnpEgKY it's impossible not to smile after hearing this :P
+1 for Chinese Electric Batman