Post your useful tips here. 
My tip - pay close attention to the kill feed, especially when you are about to use your ultimate.
If your team has lost 2 or more players, strongly consider whether or not using your ultimate ability is worth it. More often than not you should retreat, group up, and use your ultimate later once you have all six members in the next fight.
Be vocal, either in voice chat or with prompts. Call outs are especially important to deal with flankers or help pick off someone with low health. It’s also good to coordinate ults, so your team doesn’t waste all their ults in a single team fight.
Don’t let kids in match chat or toxic players get to you. If need be, mute both chats so you can concentrate on your gameplay.
Going in tandem with the previous posts, watch your chat and teams call outs. If you’re playing Genji, your Ana or Zarya might want to combo ults and they may have called that before an attack. Or Lucio may be asking to group up, because he has a plan to use speed boost or sound barrier. Always pay attention to your teams comms so you can coordinate a successful attack/defense.
Just generally try to be more aware of your surroundings, your teammates and the enemy team. I see so many healers not actually seeing a really low hp teammate that ends up dying so unnecessarily…