A very important concept to master. You need to learn what fights to fight and what fights to run from. Trying to take an objective or kill a hero isn?t worth it if you just hopelessly die failing to do so.
1) Know your class and what classes you can kill. If you?re someone like Diablo who has almost no damage then you need to not sit and fight an assassin like Illidan. You can try to CC them into a tower or your allies but don?t sit there trading blows because they?ll out race you. If you?re more of a sustained damage type of hero like Valla then you shouldn?t try to take out someone like Tassadar that has excellent damage mitigation.
2) Don?t over extend. Pushing a lane or fighting in a team fight is great when you have the advantage but you need to realize when you?re losing your advantage. If you?re pushing into their base because their team is dead then keep and eye on their res timers so you know when you need to fall back. If an enemy hero is at 1% health but is making you chase them then you need to not chase them into their base or team to just die. Keep constant track of your advantage and use it as long as you have it, fall back when you lose it. Always measure the risk with the reward in every engagement you do.
3) Keep an eye on team levels. If you?re about to hit level 10 and are in the middle of a team fight hold out for a little longer so you can wipe them with your ultimates. On the flip side you have to be wary of being in an engagement when the enemy team is about to hit level 10 even if you?re winning the fight. If you think you can kill most of them before they get level 10 then go for it, if you can?t then you need to fall back.
4) Don?t over do it on objectives. If you?re fighting over an objective like a tribute then you need to know when to cut your losses. If your team is scattered when the objective comes up then don?t just blindly rush in. Dying one by one trying to get an objective isn?t smart. Only go into a fight like that if you know you can stall long enough for your team to come and support you, otherwise let the enemy have it.
I?ve seen lots of games lost by people over committing to an objective. If it?s a 5 on 5 fight and your team loses 2 people with the enemy team still fully up then you need to give up and fall back.
In any of these situations you need to be smart on the abilities you use to escape. Just because you?re running away doesn?t mean you can?t use certain abilities on the run. Make use of things like Gazlowe?s Xplodium Charge or Sgt. Hammer?s 100 Caliber Blast to push enemies back while you?re running away.