I'm sure this is common with everyone: we have games with great teammates and games with horrible teammates. How do you keep yourself calm when you have horrible, unwise, idiotic, unskilled, or just plain stupid teammates?
After you die (which you probably will if you have such a team), stand up, walk around the room taking deep breaths, sit back down. Works for me.
Remind yourself that these are people you're playing with (unless you're in Dominion or TT), and that there are shitty people everywhere you go. Just take a deep breath, mute the shit-talkers, and let it go.
Because I'm aware I'm not perfect either. I make mistakes too. Why shouldn't other people? Granted, this also means I get super irritated when a person makes the same mistake over and over again. Face checking bushes, trying to duel a semi-fed vayne, focusing the tank, etc. Seriously, they say even a monkey learns after three failures.
If you are raging, don't type. It's fine to rage, but don't make it apparent to your allies.
Remember that it is just an f'ing game, you know-the kind people play for fun. So if you are raging this much over something that doesn't really matter too much in your overall life then you need to rethink why you play this game in the first place, for fun. Also, dont feed trolls it will make them worse.
