I always say Come on at the very beginning of a match. You know, you kind of boost morale even a little bit. I also booyah when a teammate does some hero type stuff and saves my ***.
I remmeber one time... I will never forget this brave squid... I don't know who he was or even if he INTENDED for this to happen... I was playing a charger and had another charger in my sights. This brave soul, jumps in front of the shot from the other charger, saving me, killing him and allowing me to shoot the opposing charger. I booyahed so hard. RIP brave warrior.