I would like to add:
When someone asks, answer.
Because I'm sick to death of
Me: "Hey, are you going Nidalee jungle?"
Me: "I called jungle way at the start and I'm technically higher pick order than you." (3rd vs 4th on blue side, for example)
Me: "If you can only play jungle I'll switch, but I need to know."
Me: "Hello?"
Me: "Okay, we have 10 seconds left and you don't have smite, I'm gonna assume you aren't jungling."
Me: locks in Rek'Sai or something
Them: (with 3 seconds left) "WTF I'M JUNGLE FUCK YOU"
They then tell the other team to report me, and attempt to steal my camps at every opportunity.
Actually, let's add another line to that:
Don't throw tantrums. If you aren't mature enough to accept when things don't go your way, you probably aren't mature enough to play an online game.