Learn when to stop fighting
This is easy, and a lot of people seems to not understand, you win a Team Fight, you can leave now, no need to stay longer, a lot of games are thrown because of this, late game TF we take some kills, my team chase one or two still alive, enemies have the time to come back and kill most of my team, now they win the game.
This and lazy backs OMG, just walk a bit more to your jungle or under a tower and base.
People feel like they won a TF, and now they cannot be killed, for some reason want to keep fighting when they low.